Hilda started jogging from the market to the nature reserve at a speed of 100 m/min at 5 a.m. Her sister started jogging from the market later. They were beside each other at 5.30 a.m. and her sister reached the nature reserve at 6 a.m. while Hilda was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the market?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.30 a.m. to 6 a.m. = 30 min

Distance between the market and the nature reserve
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

Distance that Hilda's sister had to cover to catch up with Hilda
= 30 x 100
= 3000 m

Distance that Hilda's sister travelled to the nature reserve after catching up with Hilda
= 7500 - 3000
= 4500 m

Since Hilda's sister jogged 4500 m in 30 min,
Hilda's sister average speed
= 4500 ÷ 30
= 150 m/min

Duration that Hilda's sister took to jog to the nature reserve
= 7500 ÷ 150
= 50 min

Time that Hilda's sister left the market:
50 min before 6 a.m. = 5.10 a.m.

Answer(s): 5.10 a.m.