Let x be the amount of alcohol that needs to be added

(20+x) / (100+x) = 25 / 100

Solving for x, we get 20/3

I see - the point is that we need to add X to both original water and original alchol.

Thanks!

How many liters of pure alcohol must be added to a 100-liter solution that is 20 percent alcohol in order to produce a solution that is 25 percent alcohol?
(A) 7/2
(B) 5
(C) 20/3
(D) 8
(E) 39/4

100 liter solution which is 20% alchohol ; Thus it already has 20 liters of alchohol.

Suppose x litres must be added..

Now (20 + x / 100 + x)*100 = 25

2000 + 100x = 2500 + 25x

75x = 500

Thus x = 500/75 = 20/3 litres

Hope this helps!