b=baseball cards
f=football cards
b=5f she has 5 times as many baseball cards as football cards so we multiply the football cards by 5 to make them equal
b+f=120 there are 120 football and baseball cards total
5f+f = 120 substitute 5f everywhere you see a b
6f = 120 combine like terms
f=20 divide by 6
there are 20 football cards
b=5f
b=5(20)
there are 100 baseball cards