In a coin box, the ratio of the number of pink marbles to the number of black marbles was 1 : 3. Each time, 3 pink and 6 black marbles were taken out of the coin box. A while later, only 129 black marbles were left in the coin box. What was the total number of marbles in the coin box at first?

	Pink marbles	Black marbles
Before	1x3 = 3 u	3x3 = 9 u
Change	- 3 u	- 6 u
After	0	129

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

Make the number of pink marbles at first and taken out the same.  LCM of 1 and 3 is 3.

Number of black marbles in the end
= 9 u - 6 u
= 3 u

3 u = 129

1 u = 129 ÷ 3 = 43

Total number of marbles in the coin box at first
= 3 u + 9 u
= 12 u
= 12 x 43
= 516

Answer: 516