Mrs Oh left home at 7 a.m. and drove towards the city. 2¹₄ hour later, Mr Oh left the house and drove at an average speed of 109 km/h towards the city. When Mr Oh reached his destination at 2 p.m., Mrs Oh was 10 km away from the city. Find Mrs Oh’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
¹₄ h = ¹₄ x 60 = 15 min
2¹₄ = 2 h 15 min

2 h 15 min after 7 a.m. = 9.15 a.m.

Time that Mr Oh left the house = 9.15 a.m.

Time that Mr Oh took from 9.15 a.m. to 2 p.m. to reach his destination = 4 h 45 min

1 min = ¹₆₀ h
45 min = ⁴⁵₆₀ h = ³₄ h
4 h 45 min = 4³₄ h

Distance that Mr Oh drove
= 4³₄ x 109
= 517³₄ km

Distance that Mrs Oh drove when Mr Oh reached
= 517³₄ - 10 
= 507³₄ km

Time that Mrs Oh took to travel from 7 a.m. to 2 p.m. = 7 h

Mrs Oh's average speed
= 507³₄ ÷ 7
= 72¹⁵₂₈ km/h

Answer(s): 72¹⁵₂₈ km/h