Mrs Sim left home at 9 a.m. and drove towards the city. 1²₃ hour later, Mr Sim left the house and drove at an average speed of 87 km/h towards the city. When Mr Sim reached his destination at 3 p.m., Mrs Sim was 28 km away from the city. Find Mrs Sim’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 Sim left the house = 10.40 a.m.

Time that Mr Sim took from 10.40 a.m. to 3 p.m. to reach his destination = 4 h 20 min

1 min = ¹₆₀ h
20 min = ²⁰₆₀ h = ¹₃ h
4 h 20 min = 4¹₃ h

Distance that Mr Sim drove
= 4¹₃ x 87
= 377 km

Distance that Mrs Sim drove when Mr Sim reached
= 377 - 28 
= 349 km

Time that Mrs Sim took to travel from 9 a.m. to 3 p.m. = 6 h

Mrs Sim's average speed
= 349 ÷ 6
= 58¹₆ km/h

Answer(s): 58¹₆ km/h