He has 128 more yellow marbles than grey marbles.

Then, he gives away 342 yellow marbles.

What do you think will happen?

1. He will lose the 128-marble advantage for the yellow marbles

2. It goes beyond that. The “342 - 128 = 214” indicates that he loses a further 214 yellow marbles (beyond the first 128 loss).

Now, he will have 214 fewer yellow marbles than grey marbles. That’s why “342 - 128” is used here.

“342 + 128” is only valid in this question if the person gives away 342 GREY marbles instead of yellow ones.

This is because the yellow marbles are already more than the grey marbles to begin with. The moment more grey marbles are given away, this gap widens until the number of yellow marbles become much more than the number of grey ones.