A’s average speed during the first two hours

\[= \frac{10+20}{2} = 15\]

A’s average speed during the last two hours

\[= \frac{30+40}{2} = 35\]

\[\therefore \text{Required ratio of average speed}\]

\[=15:35=3:7\]

Distance traveled by A at the end of 3rd hour

= (10 + 20 + 30) = 60 km

Distance traveled by B at the end of 3rd hour

= (20 + 25 + 30) = 75 km

Hence the distance between A and B at the end of 3rd hour

= (75 - 60) = 15 km

• Distance after 1 hour = 20 - 10 = 10 km
• Distance after 2 hours = 45 - 30 = 15 km
• Distance after 3 hours = 75 - 60 = 15 km
• Distance after 4 hours = 125 - 100 = 25 km

Hence, the distance between A and B is maximum at the end of 4 hours.