Mrs Ma left home at 9 a.m. and drove towards the city. 1⁵₁₂ hour later, Mr Ma left the house and drove at an average speed of 85 km/h towards the city. When Mr Ma reached his destination at 3 p.m., Mrs Ma was 25 km away from the city. Find Mrs Ma’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 Ma left the house = 10.25 a.m.

Time that Mr Ma 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 Ma drove
= 4⁷₁₂ x 85
= 389⁷₁₂ km

Distance that Mrs Ma drove when Mr Ma reached
= 389⁷₁₂ - 25 
= 364⁷₁₂ km

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

Mrs Ma's average speed
= 364⁷₁₂ ÷ 6
= 60⁵⁵₇₂ km/h

Answer(s): 60⁵⁵₇₂ km/h