Risa, Lynn, Gem and Eva jointly contributed to the cost of a present for their schoolmate. Risa's contribution is ¹₂ of the total contribution made by the four girls. The ratio of Eva's contribution to the total of Lynn's and Gem's contribution is 2 : 1. What is the total contribution by the four girls if Lynn's contribution is ¹₅ of Gem's contribution and the total amount contributed by Risa and Eva is $360?

Risa	Lynn	Gem	Eva	Total
1x18	1x18			2x18
	1x6		2x6
	1x1	5x1
18 u	1 u	5 u	12 u	36 u

Working backwards.

Total amount that Lynn and Gem had is the combined repeated identity.
LCM of 1 and 6 = 6

Total amount that Lynn, Gem and Eva had is the next combined repeated identity.
LCM of 1 and 18 = 18 

Total amount contributed by Risa and Eva
= 18 u + 12 u
= 30 u

30 u = 360

1 u = 360 ÷ 30 = $12

Total contribution by the four girls
= 36 u
= 36 x 12
= $432

Answer(s): $432