What length of the motorway was covered by the roadworks?

Question

Bhavna is making her usual 15 km journey to work down the motorway.
Due to roadworks, she crawls along part of the way at 20 kmph and averages the rest at 60 kmph.
If the journey takes her 21 minutes, what length of the motorway was covered by the roadworks?

Answer 1

Total time taken = (length of roadworks / 20kmph) + (remaining distance/60kmph)

(21/60)hr = (X/20) + ((15 - X)/60)
X = 3Km is the length of the road works.