Luis had $309 and Ethan had $192 at first. Each of them bought an identical skirt and an identical blazer from the same shop. The blazer cost 2 times as much as the skirt. In the end, Luis had 4 times as much money as Ethan. What was the cost of the skirt?

	Luis	Ethan	Difference
Before	$309	$192	$117
Change	- 1 skirt - 1 blazer	- 1 skirt - 1 blazer
After	4 u	1 u	3 u

Since Luis and Ethan bought the same items, the difference  in their amounts remains the same.

Difference in the amounts that they had
= 4 u - 1 u
= 3 u

3 u = 309 - 192
3 u = 117

1 u = 117 ÷ 3 = 39

Amount that Ethan had in the end
= 1 u
= $39

Cost of 1 skirt and 1 blazer
= 192 - 1 u
= 192 - 39
= $153

The blazer cost 2 times as much as the skirt.

Cost of 1 skirt = 1 p
Cost of 1 blazer = 2 p

Total cost of 1 skirt and 1 blazer
= 1 p + 2 p
= 3 p

3 p = 153

1 p = 153 ÷ 3 = 51

Cost of the skirt
= 1 p
= $51

Answer(s): $51