Question

Martin had 7 pounds of grapes left, and he gave away 2 5 of them. Explain how to use compatible numbers to estimate the amount of grapes he gave away.

Answer 1

2/5 is close to 1/2, and it is easy to find 1/2 of 7 mentally. Half of 7 is 3.5, so Martin gave away about 3.5 pounds.

Answer 2

Compatible numbers are numbers which are easy to do mental calculation. For example is when doing division like 25 and 5, 100 and 10, and 36 and 6.

First, we know that: 7 pounds=5 pounds + 2 pounds

or 7=5+2

In mathematics, the word “of” means to multiply. So:

grapes given away = 7 * (2/5)

Replacing 7 with 5 and 2:

= (5+2)*(2/5)

Using the distribution principle:

=5*(2/5) + 2*(2/5)

=2+ 4/5

=2 4/5

So grapes given away is 2 4/5 pounds.