WEBVTT mathematics/basic-math/pyo
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Welcome back to Educator.com; this lesson is on multiplying decimals.
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When you multiply decimals together, it is very different than when you are adding and subtracting decimals.
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The rules are very different; try not to get confused between the two.
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Remember when you add and subtract the decimals, you have to line up the decimal point.
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Then you add and subtract.
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Then you bring the decimal point straight down into the answer.
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When you multiply decimals, you don't worry about the decimal point at all.
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If I am going to multiply two numbers, let's say 1.1 and 6.
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I don't have to line up the decimal point; make sure you don't do that.
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All you have to do is multiply the numbers without having any consideration for the decimal point.
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I am just going to ignore it; I am going to multiply this.
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It is going to be 6; and then 6; 66.
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What you do is you count the total number of decimal places from the numbers you multiplied.
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From these two numbers, this one and this one,
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you are going to count to see how many numbers are behind the decimal point.
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Here I have one number.
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Here I have none because the decimal point is behind the 6.
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From these two numbers, from 1.1 and 6, I only have one number behind the decimal point.
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I go to my answer; I place one number behind the decimal point.
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The last number is 6; I am going to put the decimal point right there, 6.6.
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Let's do a few examples, 0.2 times 0.6.
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Again I am going to multiply the numbers without considering the decimal points.
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6 times 2 is 12.
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I don't have to multiply those 0s together; it is just 12.
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From this number, from these two numbers, the two numbers that I multiplied,
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I am going to see how many numbers I have behind the decimal point.
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From this number, I have one; from this number, I have another one.
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I have two total; I go to my answer.
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I am going to place two numbers behind the decimal point.
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It is going to become 0.12 or 0.12 or 0.12.
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I can put a 0 up here too.
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This is the whole number; we don't have any whole numbers; it is just 0.
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Another example, if Susan works 25.5 hours per week and she earns
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9 dollars and 40 cents an hour, how much does she earn in a week?
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This is how many hours she works in a week.
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This is how much she earns per hour.
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To figure out how much she earns in a whole week,
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I have to multiply how many hours she worked with how much she makes per hour.
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It is going to be 25.5 times 9 dollars and 40 cents.
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Again when I multiply these numbers, I am just going to line up the numbers.
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I don't care about the decimal point.
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25.5 times 9.40; you are just lining up the numbers.
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Let's multiply the 0; 0 times 0 is going to be all 0s.
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I am just going to move on to the next number.
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4 times 5 is 20; this is 22; 8, 9, 10.
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9 times 5 is 45; 45... that is 49; 18... that is 22.
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These are just 0s here; it is 0, 0, 7, 9, 3, and 2.
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Here is my answer when I multiply these two numbers together.
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Now I have to look at my decimal point.
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The first number, I look at these two numbers, the two numbers that I multiplied.
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I have one number here behind the decimal point and I have two numbers here.
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How many numbers do I have total?--I have three.
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I go to my answer; I count three numbers.
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Make sure that I have three numbers behind my decimal point.
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It is going to be 237.7.
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I am dealing with money here because I am trying to figure out how much she earns in a week.
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This is going to be in money; I am going to have a dollar sign.
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This becomes 239 dollars and 70 cents.
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This is how much she is going to earn in a week.
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The next example, I have 0.21 times 2.1.
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0.21 or I can read this as 21 hundredths because I have two numbers.
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This is tenths; this is hundredths.
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This would be 21 hundredths times 2.1.
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Again I am not going to line up the decimal point; 2.1 or 2 and 1 tenths.
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When I multiply this out, ignore the decimal point.
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2 times 1; write that here; that is 2; 2 times 2 is 4.
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You don't have to look at this number.
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If you want, you can just write that down.
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1 times 0 is 0; that goes there; then I add these down.
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1 plus nothing is 1; 2 plus 2 is 4; this is 4.
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Now I look at my numbers; how many numbers do I have behind decimal points?
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Here I have two; here I have another one; I have three total.
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I am going to go to my answer; I am going to count one, two, three.
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My answer, I have to make sure that there is going to be three numbers,
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the same number of numbers behind this decimal point.
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It is going to be 0.441 or 0.441.
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You can read this as 441 thousandths because I have three numbers and this is the thousandths place.
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This is my answer when I multiply 0.21 times 2.1.
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The fourth example, I am going to multiply these two numbers, 4.08 and 1.35.
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4 and 8 hundredths... again when I multiply these decimals together, I am just going to ignore my decimal points.
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I am just going to line up the numbers just like I do when I multiply whole numbers.
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This just happens to line up because there is the same number of numbers.
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Multiply this out; 8 times 5 is 40; 0, 4; this is 20.
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This is 24, 0, 2; this is 12 here; this is 8, 0, and 4.
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I add them down; 0; this is 8; this is 10.
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2, 4, 5; this is also 5.
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From here, after I multiply my two numbers, I am going to look at the actual numbers that I multiplied.
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I am going to count how many numbers I have behind decimal points.
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For this one, I have two numbers behind the decimal point.
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Here I also have two; total behind the decimal points, I have four numbers.
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You look at just these two numbers that you multiplied together.
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I have four numbers total behind decimal points; I go to my answer.
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I make sure that there is four numbers behind the decimal point.
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That is going to help me place the decimal point.
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One, two, three, four; there is four numbers; place the decimal point right there.
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Since there is four numbers behind decimal points here,
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there has to be four numbers behind the decimal point in the answer.
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It is going to be 5.5080.
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Or this number, it is a 0 at the end of a number behind the decimal point.
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I can just drop it if I want.
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Or you can leave it; it doesn't matter.
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It could be 5 and 508 thousandths.
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Either way, this can be the answer or this can be the answer.
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That is it for this lesson on multiplying decimals; thank you for watching Educator.com.