Isaac Physics Help

I am stuck on this question! I have no idea what to do first or how to answer it. Please.

In an MgSO4 solution, a current of 36μA flows. Work out how many SO4^2- ions pass a point in 15s. [Hint: assume that the Mg^2+ ions and SO4^2−ions move at the same speed in opposite directions and the movement of each type of ion is responsible for half the current]

Not a normal chemistry question, more of a physics issue.
However:
1A is one coulomb of charge per second
Hence 36 μA = 3.6 x 10^-5 Cs^-1
In 15 seconds:
Charge passed = 15 x 3.6 x 10^-5 C = 5.4 x 10^-4 C
If half of the charge is per ion then each ion moves 2.7 x 10^-4 C
There are 96485 coulombs in 1F and this is equivalent to 6.0223 x 10²³ electrons
Hence 6.0223 x 10²³ x 2.7 x 10^-4/96485 electrons = 1.69 x 10¹⁵ electrons
But each ion carries two charges, therefore the total number of each ion passing is 8.43 x 10¹⁴