There were gold, green and brown balls in a box. Sean put another 7 gold balls and 2 brown balls in the box, then the ratio of the number of green balls to the number of brown balls became 5 : 2. Then, he doubled the number of gold balls and removed 30 brown balls. The ratio of the number of gold balls to green balls became 2 : 1. He counted and found that there were a total of 123 balls left in the box. Find the number of gold balls that he had in the end.

	Gold balls	Green balls	Brown balls	Total
Before	5 u - 7	5 u	2 u - 2
Change 1	+ 7		+ 2
After 1	1x5 = 5 u	5 u	2 u
Change 2	+ 1x5 = 5 u		- 30
After 2	2x5 = 10 u	1x5 = 5 u	2 u - 30	123

The number of green balls remains unchanged. Make the number of green balls the same. LCM of 1 and 5 is 5.

Total number of balls in the end
= 10 u + 5 u + 2 u - 30
= 17 u - 30

17 u - 30 = 123
17 u = 123 + 30
17 u = 153

1 u = 153 ÷ 17 = 9

Number of gold balls in the end
= 10 u 
= 10 x 9
= 90

Answer(s): 90