Cindy, Cathy, Gillian and Tiffany jointly contributed to the cost of a present for their schoolmate. Cindy's contribution is ¹₃ of the total contribution made by the four girls. The ratio of Tiffany's contribution to the total of Cathy's and Gillian's contribution is 5 : 3. What is the total contribution by the four girls if Cathy's contribution is ¹₄ of Gillian's contribution and the total amount contributed by Cindy and Tiffany is $900?

Cindy	Cathy	Gillian	Tiffany	Total
1x20	2x20			3x20
	3x5		5x5
	1x3	4x3
20 u	3 u	12 u	25 u	60 u

Working backwards.

Total amount that Cathy and Gillian had is the combined repeated identity.
LCM of 3 and 5 = 15

Total amount that Cathy, Gillian and Tiffany had is the next combined repeated identity.
LCM of 2 and 20 = 40 

Total amount contributed by Cindy and Tiffany
= 20 u + 25 u
= 45 u

45 u = 900

3 u = 900 ÷ 45 = $20

Total contribution by the four girls
= 60 u
= 60 x 20
= $1200

Answer(s): $1200