C1V1 = C2V2
C1 will be the concentration of the original solution,
V1 will be the volume of the original solution,
C2 will be the concentration o the diluted solution, and
V2 will be the volume of diluted solution.
(25%)(20L) = (10%)V2
V2 = (25%)(20L) / (10%)
Therefore, V2 = 50L
Good luck :)
