Mrs Kong left home at 7 a.m. and drove towards the city. 3⁵₁₂ hour later, Mr Kong left the house and drove at an average speed of 108 km/h towards the city. When Mr Kong reached his destination at 3 p.m., Mrs Kong was 24 km away from the city. Find Mrs Kong’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₁₂ h = ⁵₁₂ x 60 = 25 min
3⁵₁₂ = 3 h 25 min

3 h 25 min after 7 a.m. = 10.25 a.m.

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

Time that Mr Kong 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 Kong drove
= 4⁷₁₂ x 108
= 495 km

Distance that Mrs Kong drove when Mr Kong reached
= 495 - 24 
= 471 km

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

Mrs Kong's average speed
= 471 ÷ 8
= 58⁷₈ km/h

Answer(s): 58⁷₈ km/h