Mrs Chong left home at 8 a.m. and drove towards the city. 2¹₂ hour later, Mr Chong left the house and drove at an average speed of 85 km/h towards the city. When Mr Chong reached his destination at 3 p.m., Mrs Chong was 24 km away from the city. Find Mrs Chong’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₂ h = ¹₂ x 60 = 30 min
2¹₂ = 2 h 30 min

2 h 30 min after 8 a.m. = 10.30 a.m.

Time that Mr Chong left the house = 10.30 a.m.

Time that Mr Chong took from 10.30 a.m. to 3 p.m. to reach his destination = 4 h 30 min

1 min = ¹₆₀ h
30 min = ³⁰₆₀ h = ¹₂ h
4 h 30 min = 4¹₂ h

Distance that Mr Chong drove
= 4¹₂ x 85
= 382¹₂ km

Distance that Mrs Chong drove when Mr Chong reached
= 382¹₂ - 24 
= 358¹₂ km

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

Mrs Chong's average speed
= 358¹₂ ÷ 7
= 51³₁₄ km/h

Answer(s): 51³₁₄ km/h