easy, think like this 5000iu split into 3 ml=1,666 iu per ml or 100 units, so cut that in half, thats 833iu per 1/2 ml or 50units, so devide that in half again, thats 416iu per 1/4 ml or 25 units so devide that in half again, thats 208iu per 1/8 ml or 12.5units
or take like 416iu devide it into 25 and you got 16.6, thats how many iu are in every unit.