Mrs Gan left home at 7 a.m. and drove towards the city. 2²₃ hour later, Mr Gan left the house and drove at an average speed of 108 km/h towards the city. When Mr Gan reached his destination at 3 p.m., Mrs Gan was 25 km away from the city. Find Mrs Gan’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
²₃ h = ²₃ x 60 = 40 min
2²₃ = 2 h 40 min

2 h 40 min after 7 a.m. = 9.40 a.m.

Time that Mr Gan left the house = 9.40 a.m.

Time that Mr Gan took from 9.40 a.m. to 3 p.m. to reach his destination = 5 h 20 min

1 min = ¹₆₀ h
20 min = ²⁰₆₀ h = ¹₃ h
5 h 20 min = 5¹₃ h

Distance that Mr Gan drove
= 5¹₃ x 108
= 576 km

Distance that Mrs Gan drove when Mr Gan reached
= 576 - 25 
= 551 km

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

Mrs Gan's average speed
= 551 ÷ 8
= 68⁷₈ km/h

Answer(s): 68⁷₈ km/h