Question 148868
x=number of 3 cent stamps
y=number of 8 cent stamps
z=number of 15 cent stamps


3x+8y+15z=247
y=3x-1
z=3x-7


Plug in the values for y and z in the original equation
3x+8(3x-1)+15(3x-7)=247


Distribute
3x+24x-8+45x-105=247


Combine like terms
72x-113=247


Add 113 to both sides
72x=360


Divide both sides by 72
x=5


Those are 3 cent stamps. You want 8 cent stanps, so
y=number of 8 cent stamps
y=3x-1


y=3(5)-1
y=15-1
y=14


There are 14 eight cent stamps in the collection.