Sabrina, Gwen, Tiffany and Sarah jointly contributed to the cost of a present for their classmate. Sabrina's contribution is ¹₅ of the total contribution made by the four girls. The ratio of Sarah's contribution to the total of Gwen's and Tiffany's contribution is 5 : 3. What is the total contribution by the four girls if Gwen's contribution is ¹₅ of Tiffany's contribution and the total amount contributed by Sabrina and Sarah is $42?

Sabrina	Gwen	Tiffany	Sarah	Total
1x4	4x4			5x4
	3x2		5x2
	1x1	5x1
4 u	1 u	5 u	10 u	20 u

Working backwards.

Total amount that Gwen and Tiffany had is the combined repeated identity.
LCM of 3 and 6 = 6

Total amount that Gwen, Tiffany and Sarah had is the next combined repeated identity.
LCM of 4 and 4 = 16 

Total amount contributed by Sabrina and Sarah
= 4 u + 10 u
= 14 u

14 u = 42

3 u = 42 ÷ 14 = $3

Total contribution by the four girls
= 20 u
= 20 x 3
= $60

Answer(s): $60