1. Answer=66cards
Lets use the variable 'x' to represent the number of baseball cards in Keith's collection that he started with. The following equation can be made.
(x+6)/2=36
multiply both sides by 2
x+6=72
subtract 6 from both sides
x=66 cards
2. Answer=$12
Lets use the variable 'a' to represent Sara's allowance. The following equation can be made:
(a/2)+7=13
subtract 7 from both sides
a/2=6
multiply both sides by 2
a=$12