Mrs Ng left home at 7 a.m. and drove towards the city. 1⁵₆ hour later, Mr Ng left the house and drove at an average speed of 103 km/h towards the city. When Mr Ng reached his destination at 1 p.m., Mrs Ng was 21 km away from the city. Find Mrs Ng’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₆ h = ⁵₆ x 60 = 50 min
1⁵₆ = 1 h 50 min

1 h 50 min after 7 a.m. = 8.50 a.m.

Time that Mr Ng left the house = 8.50 a.m.

Time that Mr Ng took from 8.50 a.m. to 1 p.m. to reach his destination = 4 h 10 min

1 min = ¹₆₀ h
10 min = ¹⁰₆₀ h = ¹₆ h
4 h 10 min = 4¹₆ h

Distance that Mr Ng drove
= 4¹₆ x 103
= 429¹₆ km

Distance that Mrs Ng drove when Mr Ng reached
= 429¹₆ - 21 
= 408¹₆ km

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

Mrs Ng's average speed
= 408¹₆ ÷ 6
= 68¹₃₆ km/h

Answer(s): 68¹₃₆ km/h