IMAT 2012 Q33 [Train Tunnels]

A regular train service operates between Jayford and Kayton, a 16 km journey which takes 19 minutes. The trains travel at a constant speed of 60 km per hour in both directions except through a tunnel, where they are limited to 20 km per hour. Trains travelling towards Kayton enter the tunnel 4 km after setting off from Jayford.

How long is the tunnel?

A. 4.5 km
B. 2.5 km
C. 1.5 km
D. 3.5 km
E. 0.5 km