Quiz on percentage of Quantity

Solution:

Water  = 20% of the mixture of 80 litres = 20 * (80 / 100)= 16

Spirit = total quantity - Water = 80 - 16 = 64

let 'x' be the amount of water need to be added to make it 25%

then 

((80+x)*25)/100 + 64 = 80 + x

=>(80+x)/4 + 64 = 80 + x

=>(80+x)/4  = 80 - 64+ x

=>(80+x)/4  = 16 + x

=>(80+x)  = 64+ 4x

=>80 - 64  = 4x - x

=>3x  = 16

=>x  = 16/3

There fore  

The amount of water need to be added to make mixture 25% = 5 1/3 litres.