Since we are talking about the unknown cost of a shirt AND a sweater, we are dealing with 2 unknowns. However, we can only have one unknown in a single equation or we cannot solve it. The cost of the sweater is based on the cost of the shirt, so the shirt will be our "main" unknown.
Cost of the shirt: x
Since the sweater is $5 more than (this is addition) twice (that is 2 times) the cost of the shirt, the expression for the sweater is 2x + 5
The cost of both is (equals) 65.
x + 2x + 5 = 65 and
3x + 5 = 65 and
3x = 60 so
x = 20
The shirt cost $20 so the sweater had to cost 65 - 20 = 45
