Vanessa, Betty, Tina and Risa jointly contributed to the cost of a present for their brother. Vanessa's contribution is ²₉ of the total contribution made by the four girls. The ratio of Risa's contribution to the total of Betty's and Tina's contribution is 5 : 2. What is the total contribution by the four girls if Betty's contribution is ¹₃ of Tina's contribution and the total amount contributed by Vanessa and Risa is $28?

Vanessa	Betty	Tina	Risa	Total
2x2	7x2			9x2
	2x2		5x2
	1x1	3x1
4 u	1 u	3 u	10 u	18 u

Working backwards.

Total amount that Betty and Tina had is the combined repeated identity.
LCM of 2 and 4 = 4

Total amount that Betty, Tina and Risa had is the next combined repeated identity.
LCM of 7 and 2 = 14 

Total amount contributed by Vanessa and Risa
= 4 u + 10 u
= 14 u

14 u = 28

2 u = 28 ÷ 14 = $2

Total contribution by the four girls
= 18 u
= 18 x 2
= $36

Answer(s): $36