Mrs Zhang left home at 7 a.m. and drove towards the city. 1²₃ hour later, Mr Zhang left the house and drove at an average speed of 110 km/h towards the city. When Mr Zhang reached his destination at 3 p.m., Mrs Zhang was 17 km away from the city. Find Mrs Zhang’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 7 a.m. = 8.40 a.m.

Time that Mr Zhang left the house = 8.40 a.m.

Time that Mr Zhang took from 8.40 a.m. to 3 p.m. to reach his destination = 6 h 20 min

1 min = ¹₆₀ h
20 min = ²⁰₆₀ h = ¹₃ h
6 h 20 min = 6¹₃ h

Distance that Mr Zhang drove
= 6¹₃ x 110
= 696²₃ km

Distance that Mrs Zhang drove when Mr Zhang reached
= 696²₃ - 17 
= 679²₃ km

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

Mrs Zhang's average speed
= 679²₃ ÷ 8
= 84²³₂₄ km/h

Answer(s): 84²³₂₄ km/h