Risa, Fanny, Jean and Cathy jointly contributed to the cost of a present for their brother. Risa's contribution is ²₃ of the total contribution made by the four girls. The ratio of Cathy's contribution to the total of Fanny's and Jean's contribution is 2 : 1. What is the total contribution by the four girls if Fanny's contribution is ²₃ of Jean's contribution and the total amount contributed by Risa and Cathy is $720?

Risa	Fanny	Jean	Cathy	Total
2x15	1x15			3x15
	1x5		2x5
	2x1	3x1
30 u	2 u	3 u	10 u	45 u

Working backwards.

Total amount that Fanny and Jean had is the combined repeated identity.
LCM of 1 and 5 = 5

Total amount that Fanny, Jean and Cathy had is the next combined repeated identity.
LCM of 1 and 15 = 15 

Total amount contributed by Risa and Cathy
= 30 u + 10 u
= 40 u

40 u = 720

1 u = 720 ÷ 40 = $18

Total contribution by the four girls
= 45 u
= 45 x 18
= $810

Answer(s): $810