Mrs Ang left home at 8 a.m. and drove towards the city. 1⁵₁₂ hour later, Mr Ang left the house and drove at an average speed of 91 km/h towards the city. When Mr Ang reached his destination at 2 p.m., Mrs Ang was 28 km away from the city. Find Mrs Ang’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 8 a.m. = 9.25 a.m.

Time that Mr Ang left the house = 9.25 a.m.

Time that Mr Ang took from 9.25 a.m. to 2 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 Ang drove
= 4⁷₁₂ x 91
= 417¹₁₂ km

Distance that Mrs Ang drove when Mr Ang reached
= 417¹₁₂ - 28 
= 389¹₁₂ km

Time that Mrs Ang took to travel from 8 a.m. to 2 p.m. = 6 h

Mrs Ang's average speed
= 389¹₁₂ ÷ 6
= 64⁶¹₇₂ km/h

Answer(s): 64⁶¹₇₂ km/h