@dulqar, 1/10^{9 }= 1 nsec = 1*10^{-9} sec.
so first term will become 30* 10^{-3} * 10^{-9} (because of 1/10^{9})= .03nsec
in second term 300* 10^{-9 }is given so u can take it as 300nsec
so it will become 300nsec*(1 - (1/10^{9})) == 300nsec*(1 - 10*^{9})
so the -ve term is quite small so we can neglect this term so addtion of 1st and 2ns term is 300nsec + .03nsec