I keep the marbles in 9 boxes each one as much, if I take all the marbles of the 6 boxes and distributed as much to each of two others, the contents of boxes, each grew 10 marbles. How much my marbles?
Suppose in each box there are A marbles.
9A = 3 x (A + 10)
9A = 3A + 30
6A = 30
A = 5
The number of marbles that I have is 9 x A = 9 x 5 = 45 marbles.