Dilution with 3 Different Percentages
OK, for what it's worth:
Let $x_f$ be the final solution mass and $x$ be the mass of the 25% solution. 
The dissolved mass can be defined by
$$0.3x_f~=~40+0.25x$$

The solution mass is:
$$x_f=100+x$$

Substitute $x_f$ into the first equation and solve. You should get the 25% solution mass to be added and then solve the easy solution mass equation to check your answer.