suppose man catch the bus in time t
distance travelled by bus is
s = ut + 1/2at^2
s = 1/2(1)t^2 = 0.5t^2
the man started running from 40 m behaind when the bus starts. so in the time t man has to run S+40
S+40 = ut = 9t
0.5t^2 + 40 = 9t
t = 8s or 10sec
we need minimum time so the man will catch the bus in 8sec .