10 L of a 25% acid solution contains 0.25 * (10 L) = 2.5 L of acid.
Adding x L of pure water dilutes the solution to a concentration of 10%, such that
(2.5 L)/(10 L + x L) = 0.10
Solve for x :
2.5 = 0.10 * (10 + x)
2.5 = 1 + 0.10x
1.5 = 0.10x
15 = x
so 15 L of pure water are needed.
