Mrs Lee left home at 8 a.m. and drove towards the city. 2⁷₁₂ hour later, Mr Lee left the house and drove at an average speed of 91 km/h towards the city. When Mr Lee reached his destination at 3 p.m., Mrs Lee was 17 km away from the city. Find Mrs Lee’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁷₁₂ h = ⁷₁₂ x 60 = 35 min
2⁷₁₂ = 2 h 35 min

2 h 35 min after 8 a.m. = 10.35 a.m.

Time that Mr Lee left the house = 10.35 a.m.

Time that Mr Lee took from 10.35 a.m. to 3 p.m. to reach his destination = 4 h 25 min

1 min = ¹₆₀ h
25 min = ²⁵₆₀ h = ⁵₁₂ h
4 h 25 min = 4⁵₁₂ h

Distance that Mr Lee drove
= 4⁵₁₂ x 91
= 401¹¹₁₂ km

Distance that Mrs Lee drove when Mr Lee reached
= 401¹¹₁₂ - 17 
= 384¹¹₁₂ km

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

Mrs Lee's average speed
= 384¹¹₁₂ ÷ 7
= 54⁸³₈₄ km/h

Answer(s): 54⁸³₈₄ km/h