A maths group from Devonshire Primary School sent us a solution for this problem:
The 10am coach will see the 8.40, 9.00, 9.20, 9.40, 10, 10.20, 10.40, 11, 11.20 coach on its way to Betaville. Therefore it sees 9 coaches. We know this because they are travelling at the same speed and on the same path.
Thank you to you all.