Question 562273
Hi, let x be the volume of acid to be added.  Let V be the volume of the new solution and V_a be the volume of the acid in the new solution.
Then,
V=40+x
V_a=(0.25)*40+(1.00)*x=10+x  (since x is purely acid)
(0.40)V=V_a
To solve for x, we substitute to get,
(0.40)(40+x)=10+x
So by solving this last equation for x, we find x=10