Mrs Fong left home at 7 a.m. and drove towards the city. 1³₄ hour later, Mr Fong left the house and drove at an average speed of 107 km/h towards the city. When Mr Fong reached his destination at 1 p.m., Mrs Fong was 21 km away from the city. Find Mrs Fong’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
³₄ h = ³₄ x 60 = 45 min
1³₄ = 1 h 45 min

1 h 45 min after 7 a.m. = 8.45 a.m.

Time that Mr Fong left the house = 8.45 a.m.

Time that Mr Fong took from 8.45 a.m. to 1 p.m. to reach his destination = 4 h 15 min

1 min = ¹₆₀ h
15 min = ¹⁵₆₀ h = ¹₄ h
4 h 15 min = 4¹₄ h

Distance that Mr Fong drove
= 4¹₄ x 107
= 454³₄ km

Distance that Mrs Fong drove when Mr Fong reached
= 454³₄ - 21 
= 433³₄ km

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

Mrs Fong's average speed
= 433³₄ ÷ 6
= 72⁷₂₄ km/h

Answer(s): 72⁷₂₄ km/h