Our main goal is to estimate the product of two decimals. The key idea behind it is to approximate each number to a reasonable number that is easier to calculate. Take for example the product of 15.5 and 6.5. We could approximate each number to the closes integer (16 and 7). So we can multiply 16 times 7 to get an estimate (112).
However, 15.5 is between 10 and 20 which is are numbers that are easy to multiply by while calculating. So you can aproximate 15.5 to 20. Also, we can aproximate 6.5 to 5, so another estimate of this product is 20 times 5, which is 100.
In this case, 15.5 times 6.5 is 100.75, so we can see that the estimates are relatively close to the real value
