Mrs Lau left home at 9 a.m. and drove towards the city. 1⁷₁₂ hour later, Mr Lau left the house and drove at an average speed of 89 km/h towards the city. When Mr Lau reached his destination at 2 p.m., Mrs Lau was 28 km away from the city. Find Mrs Lau’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁷₁₂ h = ⁷₁₂ x 60 = 35 min
1⁷₁₂ = 1 h 35 min

1 h 35 min after 9 a.m. = 10.35 a.m.

Time that Mr Lau left the house = 10.35 a.m.

Time that Mr Lau took from 10.35 a.m. to 2 p.m. to reach his destination = 3 h 25 min

1 min = ¹₆₀ h
25 min = ²⁵₆₀ h = ⁵₁₂ h
3 h 25 min = 3⁵₁₂ h

Distance that Mr Lau drove
= 3⁵₁₂ x 89
= 304¹₁₂ km

Distance that Mrs Lau drove when Mr Lau reached
= 304¹₁₂ - 28 
= 276¹₁₂ km

Time that Mrs Lau took to travel from 9 a.m. to 2 p.m. = 5 h

Mrs Lau's average speed
= 276¹₁₂ ÷ 5
= 55¹³₆₀ km/h

Answer(s): 55¹³₆₀ km/h