By moving one marble from B to A. It means B loses one marble and A gains one marble. This causes the difference between A and B to increase by two. (If u cannot see that, here's an example if B has 3 and A has 5 if u move one marble from B to A. B has 2 and A has 6. The difference between the two boxes increases from 2 to 4. It increases by 2!)
So now that we know, the difference increases by 2.
Difference between 2 boxes now = difference before + 2 = 2+2 =4
A has 4 more than B and is twice the size of B. Therefore one unit is 4.
A has 2 units = 4*2 = 8

2)
So they will have the same number means... 15/3 = 5
So this question want us to work backwards. So if marbles were given to A, we want to minus it away. And if marbles were taken away from A we want to add it back. So let's start from A gaining one marble from C.
5 - 1 = 4
Then A giving B 3. So 4 +3 = 7