Mrs Song left home at 7 a.m. and drove towards the city. 2⁵₁₂ hour later, Mr Song left the house and drove at an average speed of 91 km/h towards the city. When Mr Song reached his destination at 1 p.m., Mrs Song was 10 km away from the city. Find Mrs Song’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₁₂ h = ⁵₁₂ x 60 = 25 min
2⁵₁₂ = 2 h 25 min

2 h 25 min after 7 a.m. = 9.25 a.m.

Time that Mr Song left the house = 9.25 a.m.

Time that Mr Song took from 9.25 a.m. to 1 p.m. to reach his destination = 3 h 35 min

1 min = ¹₆₀ h
35 min = ³⁵₆₀ h = ⁷₁₂ h
3 h 35 min = 3⁷₁₂ h

Distance that Mr Song drove
= 3⁷₁₂ x 91
= 326¹₁₂ km

Distance that Mrs Song drove when Mr Song reached
= 326¹₁₂ - 10 
= 316¹₁₂ km

Time that Mrs Song took to travel from 7 a.m. to 1 p.m. = 6 h

Mrs Song's average speed
= 316¹₁₂ ÷ 6
= 52⁴⁹₇₂ km/h

Answer(s): 52⁴⁹₇₂ km/h