three note pads and one pencil cost 70 cents. One note pad and three pencils cost 50 cents. How much do two pencils and two note pads cost?
3n+p = 70
n+3p = 50
add them together and you have
4n+4p = 120
so,
2n+2p = 60
n+3p = 50
add them together and you have
4n+4p = 120
so,
2n+2p = 60