Natalie started jogging from the grocery store to the garden at a speed of 110 m/min at 5 a.m. Her sister started jogging from the grocery store later. They were beside each other at 5.40 a.m. and her sister reached the garden at 6 a.m. while Natalie 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 5 a.m. to 6 a.m. = 1 h
From 5.40 a.m. to 6 a.m. = 20 min

Distance between the grocery store and the garden
= 60 x 110 + 1800
= 6600 + 1800
= 8400 m

Distance that Natalie's sister had to cover to catch up with Natalie
= 40 x 110
= 4400 m

Distance that Natalie's sister travelled to the garden after catching up with Natalie
= 8400 - 4400
= 4000 m

Since Natalie's sister jogged 4000 m in 40 min,
Natalie's sister average speed
= 4000 ÷ 20
= 200 m/min

Duration that Natalie's sister took to jog to the garden
= 8400 ÷ 200
= 42 min

Time that Natalie's sister left the grocery store:
42 min before 6 a.m. = 5.18 a.m.

Answer(s): 5.18 a.m.