Distance D1 = 15 km Speed V1 = 30 kmph Duration T1 = 15/30 = 0.50 hrs
Distance D2 = 75 km speed V2 = 50kmph duration T2 = 75/50 = 1.5 hrs
Distance D3 = 10 km speed V3 = 20 kmph duration T3 = 10/20 = 0.5 hrs total D = 100 km total = 2 hours
average speed = V = total distance / total time duration = 100 km / 2 hrs = 50 kmph
We can also do this by using the formula:
If you can use the formula, that is fine. Other wise do that in the first method.