by adding distilled water to a 15% acid solution, the acid becomes more diluted and percentage of acid is lowered.
we can use the following formula
c1v1 = c2v2
where c1 is concentration in percentage form and v1 is volume of the concentrated solution
c2 is concentration and v2 is volume of the diluted solution
substituting these values in the formula
15% x 20 L = C x 30 L
C = 10%
concentration of acid in new solution is 10%