Paul had $270 and Billy had $100 at first. Each of them bought an identical skirt and an identical shirt from the same shop. The shirt cost 2 times as much as the skirt. In the end, Paul had 3 times as much money as Billy. What was the cost of the skirt?

	Paul	Billy	Difference
Before	$270	$100	$170
Change	- 1 skirt - 1 shirt	- 1 skirt - 1 shirt
After	3 u	1 u	2 u

Since Paul and Billy bought the same items, the difference  in their amounts remains the same.

Difference in the amounts that they had
= 3 u - 1 u
= 2 u

2 u = 270 - 100
2 u = 170

1 u = 170 ÷ 2 = 85

Amount that Billy had in the end
= 1 u
= $85

Cost of 1 skirt and 1 shirt
= 100 - 1 u
= 100 - 85
= $15

The shirt cost 2 times as much as the skirt.

Cost of 1 skirt = 1 p
Cost of 1 shirt = 2 p

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

3 p = 15

1 p = 15 ÷ 3 = 5

Cost of the skirt
= 1 p
= $5

Answer(s): $5