In a coin box, the ratio of the number of grey balls to the number of purple balls was 1 : 3. Each time, 3 grey and 7 purple balls were taken out of the coin box. A while later, only 146 purple balls were left in the coin box. What was the total number of balls in the coin box at first?

	Grey balls	Purple balls
Before	1x3 = 3 u	3x3 = 9 u
Change	- 3 u	- 7 u
After	0	146

Since there is no grey balls left, this means that the number of grey marbles at first is the same as the number of grey marbles taken out.  

Make the number of grey balls at first and taken out the same.  LCM of 1 and 3 is 3.

Number of purple balls in the end
= 9 u - 7 u
= 2 u

2 u = 146

1 u = 146 ÷ 2 = 73

Total number of balls in the coin box at first
= 3 u + 9 u
= 12 u
= 12 x 73
= 876

Answer: 876