At first, Tom, Harry and Andy had the same amount of money. After Andy spent some of his money, Tom spent ³₇ of his money and Harry spent ²₃ of his money, Tom had $50 more than Harry. In the end, the total amount of money the three boys had left was $310. How much money did Andy spend?

	Tom	Harry	Andy	Total
Before	7x3 =  21 u	3x7 = 21 u	21 u
Change	- 3x3 = - 9 u	- 2x7 = - 14 u	- ?
After	4x3 = 12 u	1x7 = 7 u		$310

The amount that Tom and Harry each had at first is the same.  Make the amount that they each had the same.  LCM of 7 and 3 is 21.

Amount that Tom had more than Harry in the end
= 12 u - 7 u
= 5 u

5 u = 50

1 u = 50 ÷ 5 = 10

Amount that Tom and Harry had in the end
= 12 u + 7 u
= 19 u
= 19 x 10
= $190

Amount that Andy had in the end
= 310 - 190
= $120

Amount that Andy had at first
= 21 u
= 21 x 10
= $210

Amount that Andy spent
= 210 - 120
= $90

Answer(s): $90