Mrs Leow left home at 8 a.m. and drove towards the city. 1⁵₁₂ hour later, Mr Leow left the house and drove at an average speed of 108 km/h towards the city. When Mr Leow reached his destination at 3 p.m., Mrs Leow was 18 km away from the city. Find Mrs Leow’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₁₂ h = ⁵₁₂ x 60 = 25 min
1⁵₁₂ = 1 h 25 min

1 h 25 min after 8 a.m. = 9.25 a.m.

Time that Mr Leow left the house = 9.25 a.m.

Time that Mr Leow took from 9.25 a.m. to 3 p.m. to reach his destination = 5 h 35 min

1 min = ¹₆₀ h
35 min = ³⁵₆₀ h = ⁷₁₂ h
5 h 35 min = 5⁷₁₂ h

Distance that Mr Leow drove
= 5⁷₁₂ x 108
= 603 km

Distance that Mrs Leow drove when Mr Leow reached
= 603 - 18 
= 585 km

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

Mrs Leow's average speed
= 585 ÷ 7
= 83⁴₇ km/h

Answer(s): 83⁴₇ km/h