Motion and Time

Boojho walks to his school which is at a distance of 3 km from
his home in 30 minutes. On reaching he finds that the school
is closed and comes back by a bicycle with his friend and
reaches home in 20 minutes. His average speed in km/h is
(a) 8.3
(b) 7.2
(c) 5
(d) 3.6