Xuan started jogging from the grocery store to the park at a speed of 108 m/min at 6 a.m. Her sister started jogging from the grocery store later. They were beside each other at 6.35 a.m. and her sister reached the park at 7 a.m. while Xuan was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the grocery store?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.35 a.m. to 7 a.m. = 25 min

Distance between the grocery store and the park
= 60 x 108 + 1800
= 6480 + 1800
= 8280 m

Distance that Xuan's sister had to cover to catch up with Xuan
= 35 x 108
= 3780 m

Distance that Xuan's sister travelled to the park after catching up with Xuan
= 8280 - 3780
= 4500 m

Since Xuan's sister jogged 4500 m in 35 min,
Xuan's sister average speed
= 4500 ÷ 25
= 180 m/min

Duration that Xuan's sister took to jog to the park
= 8280 ÷ 180
= 46 min

Time that Xuan's sister left the grocery store:
46 min before 7 a.m. = 6.14 a.m.

Answer(s): 6.14 a.m.