Question

Estimate 7.12+2.501+9.2

Answer 1

A quick estimate is to add the integer (whole number) parts together, and treat 9 as 10. That comes to 19. We can add the numbers together properly, knowing that the answer will be around 19.
To make the addition easier we can add trailing zeroes to the shorter decimals so that they all have the same number of decimal places, 3:
7.120+
2.501+
9.200
______
18.821
Yes, the estimate was right!