For neutralization of acid by a base (or vice versa), the equation should be used.
M₁V₁ = M₂V₂
where M's are the molarity and the Vs are the volume. Substituting the known values,
(0.150M)(25) = M₂(15 mL)
The value of M₂ from the equation is equal to 0.25M. Thus, the concentration of the acid is 0.25M.