The amount of alcohol in 400ml solution of 15% strength
=400×15100ml=60ml
Let the required amount of alcohol to be added =x ml.
It will increase the amount of the solution by x ml.
∴ The amount of alcohol now =(60+x)ml
and the amount of the solution=(400+x)ml.
Then, the strength =32%=60+x400+x=32100
⇒25x+1500=3200+8x⇒17x=1700⇒x=100.
So, the required amount of pure alcohol to be added =100ml.