Answer to a is 25 km. Reason is that 75/60= 1.25 , therefore going 1.25 km a min. Now myltiply that by 20 and you get 25 km in 20 minutes. Answer to b should be roughly 3 hours and 33 minutes. The reason is that 250 km / 75km/ hour = 3 hours 33 min.
Hope this helps
