If you're using proportions, here's one way.
The ratio is 4:7, right? Turn it into a proportion, and add in what you're given.
(4:7 is the original ratio, turn it into a fraction. 4 over 36, since that's what you're given, and 7 over x since we're trying to figure out how many stamps Paul has.)
4 7
-- --
36 x
Now cross multiply and divide. 7*36 is 252. Divide 252 by whatever number is left, 4, and get 63.
To check your answer, plug it in to the proportion.
4/36 simplifies to 1/9, and 7/63 simplifies to 1/9 as well.