10. $A 、 B$ two people bought the same number of sheets of letter paper and the same number of envelopes. $A$ put 1 sheet of letter paper in each envelope, and after using up all the envelopes, 40 sheets of letter paper were left; $B$ put 3 sheets of letter paper in each envelope, and after using up all the sheets of letter paper, 40 envelopes were left. They each bought $\qquad$ sheets of letter paper.
answer: 120