I actually did this kind of question earlier today in a math contest. It took me a while to get it, but it was quite easy.
So we basically know that:
[tex]A+B+C=75[/tex]
Now here comes the tricky part. We have to determine how many rocks there are in each box.
The easy thing about this is that we already know what will be in the boxes after the movements. It's just [tex]75÷3[/tex].
So now we know, that after the movements, each of the boxes will contain 12.
Lets do the operations on the boxes.
[tex]A-28+19=25[/tex]
[tex]B-24+28=25[/tex]
[tex]C-19+24=25[/tex]
And we find that:
[tex]A=34[/tex]
[tex]B=21[/tex]
[tex]C=20[/tex]
And from that, we get:
[tex]A=34[/tex]
Hope that helped! Enjoy.
~Cam943, Junior Moderator