Call x the number of foreign stamps and y the number of local stamps.
We write equations:
x+y=80
2x/3=y-10
Adding ten to the last equation, we have:
y=2x/3+10
Substituting this for y in the first equation, we have:
5x/3+10=80
Subtracting ten, we have:
5x/3=70
Multiplying by 3/5, we see that x=42, and y=80-42=38, so there are 42 foreign stamps and 38 local stamps.
