City B and City C is 420 km apart. Charlie left City B for City C at 10.00 a.m. travelling at an average speed of 60 km/h. Bobby left City B later than Charlie and caught up with him at 2.00 p.m. Bobby was travelling at a speed of 90 km/h.

1. At what time did Bobby leave City B?
2. How much later did Charlie arrive in City C than Bobby? Express your answer in h as a fraction or mixed number.

(a)
From 10.00 a.m. to 2.00 p.m. = 4 h

Distance that Charlie covered when Bobby caught up with Charlie
= 4 x 60
= 240 km

Time that Bobby took to travel 240 km
= 240 ÷ 90
= 2²₃ h

1 h = 60 min
²₃ h = ²₃ x 60 = 40 min
2²₃ h = 2 h 40 min

Time that Bobby left City B:
2 h 40 min before 2.00 p.m. = 11.20 a.m.

(b)
Remaining distance that they had to cover to reach City C
= 420 - 240
= 180 km

Duration that Bobby had to travel before reaching City C
= 180 ÷ 90
= 2 h

Duration that Charlie had to travel before reaching City C
= 180 ÷ 60
= 3 h

Duration that Charlie took to arrive later than Bobby in City C
= 3 - 2
= 1 h

Answer(s): (a) 11.20 a.m.; (b) 1 h