At 11 35, Charlie left Town X for Town Y, driving at a speed of 90 km/h. At 12 35, Justin also left Town X for Town Y driving at a certain speed. Both of them did not change their speed throughout the journey. At 15 35, both of them passed a school that was 180 km away from Town Y. How many minutes earlier did Justin reach Town Y than Charlie?

Distance that Charlie travelled
= 4 x 90
= 360 km

Distance between Town X and Town Y
= 360 + 180
= 540 km

Since Justin passed the school at the same time as Charlie, it means that Justin travelled 360 km in 3 hours.

Justin's speed during the journey
= 360 ÷ 3
= 120 km/h

Duration that Justin took to reach Town Y after passing the school
= 180 ÷ 120
= 1.5 h

Duration that Charlie took to reach Town Y after passing the school
= 180 ÷ 90
= 2 h

Difference between the time they took to reach Town Y
= 2 - 1.5
= 0.5 h

1 h = 60 min
0.5 h = 0.5 x 60 = 30 min

Justin reached Town Y 30 minutes earlier than Charlie.

Answer(s): 30 min