Consider reaction below:

50H3O^+(aq) + 7Cr2O7^2-(aq) +3I_2(aq) --> 6IO_4^-(aq) + 14Cr^3+(aq) + 75H2O (l)

How many electrons are transferred in this reaction?

Cr on the left is +6 each so 6*14 = 84+ and on the right is 14*3 = 42+. 84-42 = 42 e xfrd.

I on the left is zero. On the right each I is 7+ and 6*7 = 42+. 42-0 = 42e xfd.