There are some green and yellow balls in a box. If 3 green balls are removed from the box, the total number of balls left will be 7 times the number of green balls left. If 2 yellow balls are removed from the box, the total number of balls left will be 3 times the number of green balls. How many balls are there in the box?

	Case 1		Case 2
	Green balls	Yellow balls	Green balls	Yellow balls
Before	1 u + 3	6 u	1 p	2 p + 2
Change	- 3			- 2
After	1 u	6 u	1 p	2 p

Number of yellow balls in the end for Case 1
= 7 u - 1 u
= 6 u

Number of green balls in the end for Case 2
= 3 p - 1 p
= 2 p

The number of green balls at first is the same for Case 1 and Case 2.
The number of yellow balls at first is also the same for Case 1 and Case 2.

1 u + 3 = 1 p --- (1)

6 u = 2 p + 2
6 u - 2 = 2 p --- (2)

Make p the same.

(1)_x2
2 u + 6 = 2 p --- (3)

(2) = (3)
6 u - 2 = 2 u + 6
6 u - 2 u = 6 + 2
4 u = 8

1 u = 8 ÷ 4 = 2

Number of balls in the box
= (1 u + 3) + 6 u
= 7 u + 3
= (7 x 2) + 3
= 14 + 3
= 17

Answer(s): 17