Let u = marbles added to each box

Box A     [u][    128    ]

Box B     [u][42]

Now after adding the u marbles to each box, Box A is twice of Box B,  it means 2Box B = Box A

2(u+42) = u + 128

2u-u = 128 – 84

u = 44

Hence 44 marbles were added to each box. 

Video Solution can be viewed here