How to Multiply Decimals By Decimals
In this lesson, let’s learn how to multiply a decimal by another decimal. So let’s see, what are we doing? We’re multiplying a decimal which is 0.4 x 0.5. I’m going to teach two ways of doing it.
First, is by using models. So I have to grid which got a hundred squares, 10 squares across and 10 squares down. So what is 0.4? 0.4 is this much, right. 4/10 which is 40 squares. So I can draw – all I have done is color shade it 40 of the hundred squares which is 0.4.
Now, what I’m going to do is take a different color and shade 50 squares because that’s 0.5. Fifty squares would be these squares. All of these. Now, the multiplication of 0.4 x 0.5 would be, well how many squares have both colors? That’s these many, one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 16 and 20. Twenty squares which is 0.2. That’s the answer.
Another way of doing this is we use what we learned before, which is how do we multiply two numbers. We’ll be – before multiplying 0.4 and 0.5, we ignore the decimals first. We multiply 4 x 5, I got 20. Now we look at the number of decimals. Here, one digit after the decimal. Here, one digit after the decimal. One and one is two digits, so I’ll take this decimal point and we move it two digits to the left. So we move from here to over here, which means the answer is 0.20.
Either way, we arrive at the same answer which is 0.2 or 0.20. Any zeros that are at the end after a decimal point are useless anyway. So the answer is 0.4 x0.5 equals 0.2.
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