Billy had $304 and Albert had $136 at first. Each of them bought an identical cardigan and an identical shirt from the same shop. The shirt cost 4 times as much as the cardigan. In the end, Billy had 4 times as much money as Albert. What was the cost of the cardigan?

	Billy	Albert	Difference
Before	$304	$136	$168
Change	- 1 cardigan - 1 shirt	- 1 cardigan - 1 shirt
After	4 u	1 u	3 u

Since Billy and Albert 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 = 304 - 136
3 u = 168

1 u = 168 ÷ 3 = 56

Amount that Albert had in the end
= 1 u
= $56

Cost of 1 cardigan and 1 shirt
= 136 - 1 u
= 136 - 56
= $80

The shirt cost 4 times as much as the cardigan.

Cost of 1 cardigan = 1 p
Cost of 1 shirt = 4 p

Total cost of 1 cardigan and 1 shirt
= 1 p + 4 p
= 5 p

5 p = 80

1 p = 80 ÷ 5 = 16

Cost of the cardigan
= 1 p
= $16

Answer(s): $16