Keith and sarah robinson purchased 40 stamps, a mixture of 49 cent and 34 cent stamps. find the number of each type of stamp if they spent $17.35.

There are 2 equation. First let 49 cent stamps be f and 34 stamps be t. 
One equation would be f+t=40 the other one would be .49f+.34t=17.35. You can do substitution if you want or elimination. 
Put the two in line
49f+34t=1735
f+t=40> elimination
-34(f+t=40)--> solve

49f+34t=1735
-34f-34t=-1360 --->solve

            15f =375--> divide
f=25
Then put this on the first equation to find the other answer.