Mrs Sih left home at 9 a.m. and drove towards the city. 1²₃ hour later, Mr Sih left the house and drove at an average speed of 82 km/h towards the city. When Mr Sih reached his destination at 3 p.m., Mrs Sih was 28 km away from the city. Find Mrs Sih’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 Sih left the house = 10.40 a.m.

Time that Mr Sih 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 Sih drove
= 4¹₃ x 82
= 355¹₃ km

Distance that Mrs Sih drove when Mr Sih reached
= 355¹₃ - 28 
= 327¹₃ km

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

Mrs Sih's average speed
= 327¹₃ ÷ 6
= 54⁵₉ km/h

Answer(s): 54⁵₉ km/h