Mrs Leow left home at 7 a.m. and drove towards the city. 1²₃ hour later, Mr Leow left the house and drove at an average speed of 85 km/h towards the city. When Mr Leow reached his destination at 1 p.m., Mrs Leow was 23 km away from the city. Find Mrs Leow’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 7 a.m. = 8.40 a.m.

Time that Mr Leow left the house = 8.40 a.m.

Time that Mr Leow took from 8.40 a.m. to 1 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 Leow drove
= 4¹₃ x 85
= 368¹₃ km

Distance that Mrs Leow drove when Mr Leow reached
= 368¹₃ - 23 
= 345¹₃ km

Time that Mrs Leow took to travel from 7 a.m. to 1 p.m. = 6 h

Mrs Leow's average speed
= 345¹₃ ÷ 6
= 57⁵₉ km/h

Answer(s): 57⁵₉ km/h