Answer:
6 hours
Explanation:
It took him 5 hours to travel from Johannesburg to Durban at a speed of 120km/h. If he is travelling at 100km/h then he will will take more than 5 hours, since he is moving at a slower speed.
\[\dfrac{120}{100}\times 5 = 6\]
\(\therefore\) he takes 6 hours when moving at 100km/h