Zoe started jogging from the hypermart to the garden at a speed of 105 m/min at 7 a.m. Her sister started jogging from the hypermart later. They were beside each other at 7.36 a.m. and her sister reached the garden at 8 a.m. while Zoe was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the hypermart?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.36 a.m. to 8 a.m. = 24 min

Distance between the hypermart and the garden
= 60 x 105 + 1800
= 6300 + 1800
= 8100 m

Distance that Zoe's sister had to cover to catch up with Zoe
= 36 x 105
= 3780 m

Distance that Zoe's sister travelled to the garden after catching up with Zoe
= 8100 - 3780
= 4320 m

Since Zoe's sister jogged 4320 m in 36 min,
Zoe's sister average speed
= 4320 ÷ 24
= 180 m/min

Duration that Zoe's sister took to jog to the garden
= 8100 ÷ 180
= 45 min

Time that Zoe's sister left the hypermart:
45 min before 8 a.m. = 7.15 a.m.

Answer(s): 7.15 a.m.