Question

how much pure alcohol must be added to 400 ml of a 15% solution to make its strength 32 %

Answer 1

400ml has 15% alcohol i.e 60ml . This means we have 340ml of water or the liquid with which alcohol is mixed 
Let V be the volume of alcohol added to make the strength 32%
then V/340+V = 32/100. solving this we get V as 160ml

so we need to add 160 -60 = 100ml of alcohol 

A-100ml

Answer 2

400ml has 15% alcohol i.e 60ml .
This means we have 340ml of water
or the liquid with which alcohol is mixed.

 Let V be the volume of alcohol added to make the strength 32%
then,
V/340+V = 32/100. solving this we get V as 160ml

so we need to add 160 -60 = 100ml of alcohol 
= 100ml is the answer.