Vanessa started jogging from the bakery to the garden at a speed of 115 m/min at 6 a.m. Her sister started jogging from the bakery later. They were beside each other at 6.40 a.m. and her sister reached the garden at 7 a.m. while Vanessa was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the bakery?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.40 a.m. to 7 a.m. = 20 min

Distance between the bakery and the garden
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

Distance that Vanessa's sister had to cover to catch up with Vanessa
= 40 x 115
= 4600 m

Distance that Vanessa's sister travelled to the garden after catching up with Vanessa
= 8600 - 4600
= 4000 m

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

Duration that Vanessa's sister took to jog to the garden
= 8600 ÷ 200
= 43 min

Time that Vanessa's sister left the bakery:
43 min before 7 a.m. = 6.17 a.m.

Answer(s): 6.17 a.m.