Now your work is to find the minumum packet size for this network whose speed of transfer is 0.5 MBPS.
The maximum time after which collision can happen is the time t - which is the time taken to travel from one end to the other.
The collision can happen at time t = (travel time for this channel) and the damaged signal can reach back the node which sent it in t more time . Hence total time for which the channel should be listening is 2t.
Now you can calculate the size of packet using the formula sizeof packet = speed of channel * time taken to travel.
Hint : The time taken for a signal to pass through the channel is 15 seconds
Start guessing and find the minimum size