Mrs Choy left home at 8 a.m. and drove towards the city. 3¹₂ hour later, Mr Choy left the house and drove at an average speed of 109 km/h towards the city. When Mr Choy reached his destination at 3 p.m., Mrs Choy was 21 km away from the city. Find Mrs Choy’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₂ h = ¹₂ x 60 = 30 min
3¹₂ = 3 h 30 min

3 h 30 min after 8 a.m. = 11.30 a.m.

Time that Mr Choy left the house = 11.30 a.m.

Time that Mr Choy took from 11.30 a.m. to 3 p.m. to reach his destination = 3 h 30 min

1 min = ¹₆₀ h
30 min = ³⁰₆₀ h = ¹₂ h
3 h 30 min = 3¹₂ h

Distance that Mr Choy drove
= 3¹₂ x 109
= 381¹₂ km

Distance that Mrs Choy drove when Mr Choy reached
= 381¹₂ - 21 
= 360¹₂ km

Time that Mrs Choy took to travel from 8 a.m. to 3 p.m. = 7 h

Mrs Choy's average speed
= 360¹₂ ÷ 7
= 51¹₂ km/h

Answer(s): 51¹₂ km/h