Joelle buys a vest, a coat, a skirt and a windbreaker. The average cost of these items is $45. The total cost of the skirt and the windbreaker is ⁴₅ of the total cost of the vest and the coat. If the vest costs $18.70 and the windbreaker costs $9.30, find the difference in cost between the coat and the skirt.
Total cost of the 4 items
= 4 x 45
= $180

Total cost of the skirt and the windbreaker = 4 u
Total cost of the vest and the coat = 5 u

Total cost of the 4 items
= 4 u + 5 u
= 9 u

9 u = 180

1 u = 180 ÷ 9 = 20

Total cost of the skirt and the windbreaker
= 4 u
= 4 x 20
= $80

Cost of the skirt
= 80 - 9.30
= $70.70

Total cost of the vest and the coat
= 5 u
= 5 x 20
= $100

Cost of the coat
= 100 - 18.70
= $81.30

Difference in cost between the coat and the skirt
= 81.30 - 70.70
= $10.60

Answer(s): $10.60