You can set this up as a system of equations as follows. U=U.S. Stamps,
C=Canadian Stamps, P=Peter's Stamps
400*5/8=U
U+C=400
1/5*U = P
U is easy to solve for. If we multiply we get
U=250
Then we can solve for C although we don't really need to know C but we can find it anyway
C=400-250
C=150
Then we can also solve for P
1/5*250=p
p=50
If he gave 50 stamps to peter then he has
400-50 stamps left or 350