Average power dissipation in radar
OK, here's an edit that explains why the answer is 20.9 kW. The question is actually asking for radar power dissipation and not input power so, if the radar is emitting 300 kW at 60% efficiency then the input power is 500 kW and the losses are 200 kW - that's an average power loss over the whole cycle of 20 kW - add that the the 900 watts lost when the pulse isn't there and you get 20.9 kW.

Original answer:

For 90% of the time the power is 1 kW. That's an average power of 900 watts. 

For 10% of the time the power is 300 kW/0.6 = 500 kW or, an average power of 50 kW. Total average power is 50.9 kW.

I don't see how the answer can be 20.9 kW.


  here transmitted power is 300kW and efficiency of radar transmitter is
  60 %. so my power is 180W and i calculate it with duty cycle. so i am
  getting 18 kW.


You calculated that wrong - it should be 300 kW/0.6.