Mrs Leng left home at 9 a.m. and drove towards the city. 1²₃ hour later, Mr Leng left the house and drove at an average speed of 86 km/h towards the city. When Mr Leng reached his destination at 2 p.m., Mrs Leng was 18 km away from the city. Find Mrs Leng’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
²₃ h = ²₃ x 60 = 40 min
1²₃ = 1 h 40 min

1 h 40 min after 9 a.m. = 10.40 a.m.

Time that Mr Leng left the house = 10.40 a.m.

Time that Mr Leng took from 10.40 a.m. to 2 p.m. to reach his destination = 3 h 20 min

1 min = ¹₆₀ h
20 min = ²⁰₆₀ h = ¹₃ h
3 h 20 min = 3¹₃ h

Distance that Mr Leng drove
= 3¹₃ x 86
= 286²₃ km

Distance that Mrs Leng drove when Mr Leng reached
= 286²₃ - 18 
= 268²₃ km

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

Mrs Leng's average speed
= 268²₃ ÷ 5
= 53¹¹₁₅ km/h

Answer(s): 53¹¹₁₅ km/h