How to Apply Decimal Multiplication to Problems

Now let’s do some work problems on decimal multiplications. The problem says that Justin can harvest 0.7 of his family’s rice paddy in one hour. If he has worked 0.5 hours, how many much of the rice paddy has he harvested so far? Well if he can do 0.7 in an hour to do all of that, we have to multiply 0.7 x 0.5. I’m going to erase some and write again because it’s scrupling a little – so what we have to do is multiply 0.7 x 0.5.

Well, we can do this by multiplying 7 and 5. Let’s ignore the decimals for a minute. 7 x 5 is 35. Then what we do is you got one decimal here, one decimal here. Right, one decimal point, one decimal point. Which means the decimal spot is one digit from the right.

Alright, 1 + 1 is 2. So we take from their end. We two spots to the left, which gives me a 0.35. So what are we looking for? 0.7 x 0.5 equals 0.35, which make sense. Part of doing this multiplications is remembering a few tricks. If we have 0.7, which is less than 1 and 0.5, which is less than 1, the multiplication of these has to be a value which is less than 1. So that’s one way to quickly check to make sure we haven’t made a major mistake.