One day you were with your class on a picnic. During return journey from picnic spot to your school, your bus was travelling at the scheduled average speed v1=70 km/h, suddenly, it began to rain so the driver reduced the speed and drove the bus at average speed of v2=60km/h. After the rain stopped, the driver drove at the average speed of v3=75km/h to cover the remaining distance of 40 km and reached in exactly scheduled time. How long did it rain?
( type the number of minutes)