Answer: N= 375, C= 875
Step-by-step explanation:  Let Canadian stamps=C
Let International stamps=N
C=875     and        N=375
  
C - 500=N,  .70(N + C) =C
Solve the following system:
{C-500 = N |     (equation 1)
0.7 (C+N) = C |     (equation 2)
Express the system in standard form:
{-N+C = 500 |     (equation 1)
0.7 N-0.3 C = 0 |     (equation 2)
Add 0.7 × (equation 1) to equation 2:
{-N+C = 500 |     (equation 1)
0 N+0.4 C = 350 |     (equation 2)
Divide equation 2 by 0.4:
{-N+C = 500 |     (equation 1)
0. N+C = 875. |     (equation 2)
Subtract equation 2 from equation 1:
{-N+0 C = -375. |     (equation 1)
0 N+C = 875. |     (equation 2)
Multiply equation 1 by -1:
{N+0 C = 375. |     (equation 1)
0 N+C = 875. |     (equation 2)
Collect results:
Answer: |  N = 375        and             C=875