Mrs Koh left home at 9 a.m. and drove towards the city. 1¹₃ hour later, Mr Koh left the house and drove at an average speed of 100 km/h towards the city. When Mr Koh reached his destination at 1 p.m., Mrs Koh was 10 km away from the city. Find Mrs Koh’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 Koh left the house = 10.20 a.m.

Time that Mr Koh took from 10.20 a.m. to 1 p.m. to reach his destination = 2 h 40 min

1 min = ¹₆₀ h
40 min = ⁴⁰₆₀ h = ²₃ h
2 h 40 min = 2²₃ h

Distance that Mr Koh drove
= 2²₃ x 100
= 266²₃ km

Distance that Mrs Koh drove when Mr Koh reached
= 266²₃ - 10 
= 256²₃ km

Time that Mrs Koh took to travel from 9 a.m. to 1 p.m. = 4 h

Mrs Koh's average speed
= 256²₃ ÷ 4
= 64¹₆ km/h

Answer(s): 64¹₆ km/h