Question 201380
Let x=number of 20 cent stamps
Then 3x-8=number of 15 cent stamps

we'll deal in pennies
20x+15(3x-8)=400  get rid of parens
20x+45x-120=400 add 120 to each side
20x+45x-120+120=400+120  collect like terms
65x=520  divide each side by 65
x=8--------------------number of 20 cent stamps
3x-8=3*8-8=24-8=16  number of 15 cent stamps

CK
8*20+16*15=400
160+240=400
400=400

Hope this helps---ptaylor