Question

Which sum is close to 1 0.4+1/2 ,7/8+1/4,4/5+2/3 explain

Answer 1

Presuming the question is "Which sum is closest to 1". 
First, you need to either convert all the fractions to decimals or all the decimals to fractions, then add each pair. 
0.4 + 1/2 = 0.9
7/8 + 1/4 = 1.125
4/5 + 2/3 = 1.466666...
The first one is different from 1 by 0.1. The second is different by 0.125. The third is different by 0.46666.... Therefore, the closest sum is the first, or 0.4 + 1/2

0.4 + 1/2