here is my approach.
at max packets needs to travel 1000m distance so propagation delay should be
Tp=(1000)/(2x10^8) = 5 micro sec
hub having delay of 2.5 micro sec
so for collision detection it needs
Tt >= 2*Tp + hub delay
L>=B(2x5+ 2.5)
L>=125 bits
so at max we need minimum 125 bits
please tell me where i m wrong ???????