Question 296183
let beatrice have x marbles

Al will have 3x

3x-15 = 2* x

3x-2x=15

x=15

beatrice has 15 marbles

Alphonse has 45 marbles

15+45  / 2 = 30

So alphonse must give 15 marbles to beatrice so that they have the same number  30 marbles