Transmission Time(Tx)= (Size of msg or packet or data)/ ( Brand width of channel or channel capacity)
So, transmission time=1KB/10^9 bps= 8*10^-6sec
Sender utilization means the utilization of the channel by the sender
n= 1/1+2a,a=Tp/Tx for stop and wait protocol
a=15 m sec/8*10^-3 m sec= 1.875 *10^3
n= 1/1+2(1.875*10^3 )= 2.66*10^-4 sec