Mrs Chia left home at 7 a.m. and drove towards the city. 1⁷₁₂ hour later, Mr Chia left the house and drove at an average speed of 97 km/h towards the city. When Mr Chia reached his destination at 1 p.m., Mrs Chia was 24 km away from the city. Find Mrs Chia’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁷₁₂ h = ⁷₁₂ x 60 = 35 min
1⁷₁₂ = 1 h 35 min

1 h 35 min after 7 a.m. = 8.35 a.m.

Time that Mr Chia left the house = 8.35 a.m.

Time that Mr Chia took from 8.35 a.m. to 1 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 Chia drove
= 4⁵₁₂ x 97
= 428⁵₁₂ km

Distance that Mrs Chia drove when Mr Chia reached
= 428⁵₁₂ - 24 
= 404⁵₁₂ km

Time that Mrs Chia took to travel from 7 a.m. to 1 p.m. = 6 h

Mrs Chia's average speed
= 404⁵₁₂ ÷ 6
= 67²⁹₇₂ km/h

Answer(s): 67²⁹₇₂ km/h