In this lesson, let’s learn how to multiply by counting digits and we’re talking about decimals and whole numbers.

The first problem wants us to multiply 8 by 0.7. Here’s what we do. What we do is we ignore the decimals for a minute and instead of multiplying the numbers with decimals; we’ll just multiply 8×7. The answer is 56. Then what we do is we take both the values 8, it has 0 decimals because there is no decimal in here. 0.7 has one decimal. We add the two decimals to get 1 decimal. When we take the number 56, we start from the right corner and move one decimal to the left, so 8×0.7 is 5.6.

Essentially, what we did was we multiplied the numbers without the decimals. We got 56 and then how many places from the right should we place the decimal point? We add the number of decimals in both the numbers. The total is one, so we moved one place to the right.

Here’s a quick trick. 0.7 is close to 1 but it’s less than 1. If I multiply 8 by 1, I should get 8. If I multiply 8 times a number that is slightly less than one, I should get a value which is slightly less than 8. 56 obviously wouldn’t be that value, 5.6 is. I can just quickly double check my answer by looking at 8×0.7, well 0.7 is slightly less than 1, so 8×0.7 should be less than 8 but not too far less, 5.6. It seems right.

Let’s learn the same method in Part B. Drake exchanges US$65.00 for British pounds. If the exchange weight is £0.6 for each dollar, how many pounds does he receive?

He has $65.00 and for $1.00, he gets £0.6. For $65.00, he will get 65×0.6 pounds. What is 65×0.6? Well, what did we learn before? We multiply 65×6. 6×5 is 30 one, 0. I put three in the next column, 39. The answer is 390. Let’s see how many digits we move. This one has zero digits after the decimal, this one has one digit. We move one digit to the left and place the decimal point. The answer is £39.0.