Mrs Tang left home at 10 a.m. and drove towards the city. 1¹₂ hour later, Mr Tang left the house and drove at an average speed of 97 km/h towards the city. When Mr Tang reached his destination at 3 p.m., Mrs Tang was 21 km away from the city. Find Mrs Tang’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₂ h = ¹₂ x 60 = 30 min
1¹₂ = 1 h 30 min

1 h 30 min after 10 a.m. = 11.30 a.m.

Time that Mr Tang left the house = 11.30 a.m.

Time that Mr Tang took from 11.30 a.m. to 3 p.m. to reach his destination = 3 h 30 min

1 min = ¹₆₀ h
30 min = ³⁰₆₀ h = ¹₂ h
3 h 30 min = 3¹₂ h

Distance that Mr Tang drove
= 3¹₂ x 97
= 339¹₂ km

Distance that Mrs Tang drove when Mr Tang reached
= 339¹₂ - 21 
= 318¹₂ km

Time that Mrs Tang took to travel from 10 a.m. to 3 p.m. = 5 h

Mrs Tang's average speed
= 318¹₂ ÷ 5
= 63⁷₁₀ km/h

Answer(s): 63⁷₁₀ km/h