let x = amount of money shared by alice, belinda and christine
4x + 8x (alice and belinda's money)
5x + 84 (christine's money to be equal to A and B)
4x + 8x = 5x +84
4x + 8x - 5x = 84
7x = 84
divide both sides by 7
x= 12
alice: 4(12)= 48
belinda: 8(12)= 96
christine: 5(12) = 60
check:
alice + belinda = christine + 84
48 + 96 = 60 + 84
144 = 144