You're derivative is a bit off
I don't think the initial equation is correct either.
total cost for a bus to run for one hour is 90+(v²/10) .v is speed of the bus..
take "C" as the cost for bus to run 1Km.What is the speed(v ) need for bus to make the cost minimum
Here what i did:
C=[ 90+(v²/10) ] v -this is the cost for bus to run one hour.
Dc/Dv= 90 + 3v³/10
set to zero and solve
answer has to be 30 as books says