Mrs Chan left home at 7 a.m. and drove towards the city. 1¹₄ hour later, Mr Chan left the house and drove at an average speed of 85 km/h towards the city. When Mr Chan reached his destination at 3 p.m., Mrs Chan was 23 km away from the city. Find Mrs Chan’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₄ h = ¹₄ x 60 = 15 min
1¹₄ = 1 h 15 min

1 h 15 min after 7 a.m. = 8.15 a.m.

Time that Mr Chan left the house = 8.15 a.m.

Time that Mr Chan took from 8.15 a.m. to 3 p.m. to reach his destination = 6 h 45 min

1 min = ¹₆₀ h
45 min = ⁴⁵₆₀ h = ³₄ h
6 h 45 min = 6³₄ h

Distance that Mr Chan drove
= 6³₄ x 85
= 573³₄ km

Distance that Mrs Chan drove when Mr Chan reached
= 573³₄ - 23 
= 550³₄ km

Time that Mrs Chan took to travel from 7 a.m. to 3 p.m. = 8 h

Mrs Chan's average speed
= 550³₄ ÷ 8
= 68²⁷₃₂ km/h

Answer(s): 68²⁷₃₂ km/h