Yen started jogging from the church to the garden at a speed of 105 m/min at 6 a.m. Her sister started jogging from the church later. They were beside each other at 6.36 a.m. and her sister reached the garden at 7 a.m. while Yen was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the church?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.36 a.m. to 7 a.m. = 24 min

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

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

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

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

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

Time that Yen's sister left the church:
45 min before 7 a.m. = 6.15 a.m.

Answer(s): 6.15 a.m.