Mrs Chiam left home at 9 a.m. and drove towards the city. 1¹₃ hour later, Mr Chiam left the house and drove at an average speed of 83 km/h towards the city. When Mr Chiam reached his destination at 2 p.m., Mrs Chiam was 18 km away from the city. Find Mrs Chiam’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₃ h = ¹₃ x 60 = 20 min
1¹₃ = 1 h 20 min

1 h 20 min after 9 a.m. = 10.20 a.m.

Time that Mr Chiam left the house = 10.20 a.m.

Time that Mr Chiam took from 10.20 a.m. to 2 p.m. to reach his destination = 3 h 40 min

1 min = ¹₆₀ h
40 min = ⁴⁰₆₀ h = ²₃ h
3 h 40 min = 3²₃ h

Distance that Mr Chiam drove
= 3²₃ x 83
= 304¹₃ km

Distance that Mrs Chiam drove when Mr Chiam reached
= 304¹₃ - 18 
= 286¹₃ km

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

Mrs Chiam's average speed
= 286¹₃ ÷ 5
= 57⁴₁₅ km/h

Answer(s): 57⁴₁₅ km/h