Answer:
It will take him 0.5 (30 minutes) less hours if he bicycles at a speed that is 3km/h faster.
Explanation:
18 km/h x 210 minutes = 3,780
3,780/21km = 180 minutes = 3 hours
If we multiply the speed by the time we can find the distance, using that we can find the time it takes to travel the same distance with more speed (21km/s this time).
Therefore, it will take 30 minutes less if he travels 2 km/s faster or 0.5 less hours
I hope this helps!