Mrs Lee left home at 9 a.m. and drove towards the city. 1⁵₁₂ hour later, Mr Lee left the house and drove at an average speed of 108 km/h towards the city. When Mr Lee reached his destination at 3 p.m., Mrs Lee was 19 km away from the city. Find Mrs Lee’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₁₂ h = ⁵₁₂ x 60 = 25 min
1⁵₁₂ = 1 h 25 min

1 h 25 min after 9 a.m. = 10.25 a.m.

Time that Mr Lee left the house = 10.25 a.m.

Time that Mr Lee took from 10.25 a.m. to 3 p.m. to reach his destination = 4 h 35 min

1 min = ¹₆₀ h
35 min = ³⁵₆₀ h = ⁷₁₂ h
4 h 35 min = 4⁷₁₂ h

Distance that Mr Lee drove
= 4⁷₁₂ x 108
= 495 km

Distance that Mrs Lee drove when Mr Lee reached
= 495 - 19 
= 476 km

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

Mrs Lee's average speed
= 476 ÷ 6
= 79¹₃ km/h

Answer(s): 79¹₃ km/h