The present ages of a mother and her daughter add up to 40 years. Five years ago the product of their ages was 200. The daughter's present age is:

Let the daughter be $d$ years old, so the mother is $40-d$. Then $(d-5)(35-d)=200$, which simplifies to $d^2-40d+375=0$, so $d=15$ or $d=25$. The daughter is the younger of the two, so $d=15$ — five years ago the ages were 10 and 20, and $10\times 20=200$.