Mrs Tay left home at 9 a.m. and drove towards the city. 1²₃ hour later, Mr Tay left the house and drove at an average speed of 102 km/h towards the city. When Mr Tay reached his destination at 3 p.m., Mrs Tay was 19 km away from the city. Find Mrs Tay’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
²₃ h = ²₃ x 60 = 40 min
1²₃ = 1 h 40 min

1 h 40 min after 9 a.m. = 10.40 a.m.

Time that Mr Tay left the house = 10.40 a.m.

Time that Mr Tay took from 10.40 a.m. to 3 p.m. to reach his destination = 4 h 20 min

1 min = ¹₆₀ h
20 min = ²⁰₆₀ h = ¹₃ h
4 h 20 min = 4¹₃ h

Distance that Mr Tay drove
= 4¹₃ x 102
= 442 km

Distance that Mrs Tay drove when Mr Tay reached
= 442 - 19 
= 423 km

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

Mrs Tay's average speed
= 423 ÷ 6
= 70¹₂ km/h

Answer(s): 70¹₂ km/h