Alan's answer is more "mathematical" than mine, but.....
Notice that Box B picks up a net of 4 pebbles.......and since each box must contain 25 pebbles at the end, Box B must have had 21 pebbles at the start.
And Box C picks up a net of 5 pebbles......so it must had 20 at the start.
Then A must have contained 75 - 21 - 20 = 34 at the start
