Jenson had $310 and Billy had $100 at first. Each of them bought an identical shirt and an identical sweater from the same shop. The sweater cost 2 times as much as the shirt. In the end, Jenson had 4 times as much money as Billy. What was the cost of the shirt?

	Jenson	Billy	Difference
Before	$310	$100	$210
Change	- 1 shirt - 1 sweater	- 1 shirt - 1 sweater
After	4 u	1 u	3 u

Since Jenson and Billy 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 = 310 - 100
3 u = 210

1 u = 210 ÷ 3 = 70

Amount that Billy had in the end
= 1 u
= $70

Cost of 1 shirt and 1 sweater
= 100 - 1 u
= 100 - 70
= $30

The sweater cost 2 times as much as the shirt.

Cost of 1 shirt = 1 p
Cost of 1 sweater = 2 p

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

3 p = 30

1 p = 30 ÷ 3 = 10

Cost of the shirt
= 1 p
= $10

Answer(s): $10