Firstly, note that here time is given in bits not in sec. Now,
1: Both stn detect collision at t= 225.
2: after detecting collision, both will sent jam signal to inform other stn in that network about the occurrence of collision. here jam signal is 50 bits.
So both stn stop transmitting their jam signal at t= 225+50 = 275.
3: Last bit of jam signal will now propagate to other end. And Tp is given to be 225 bit time. So time at which both the jam signal will reach completely to opp end and channel is sensed idle at t= 275+225 = 500 bits ( Final answer in bits)
Now "For 10^7 bit/sec. find the time at which both nodes A and B sense idle channel?" - Since Bandwidth is given, ques is asked in units of time (sec).
So now converting answer in bits to sec :-
time at which both nodes A and B sense idle channel = 500 bits = 500 / BW = 500/ 10^7
= 50 micro sec.