(a) Without stoppage, distance travelled per hour = 60 km
With stoppage, distance travelled per hour = 40 km.
∴ Stoppage distance per hour = 60 – 40 = 20 km
Since, speed is 60 km per hour, i.e., in 60 min.(1 km – 1 min.)
Hence, stoppage time per hour = 20 min.