Consider the total power generated by magnetron as XkW. Then 40% of X is 300kW. Which means X is 500kW.
Therefore the total power dissipated at the radar when pulse is on is 500-300=200kW.
Now calculating the average value (Given the duty factor as 10%):
Avg power = 200*0.1+1*0.9 = 20.9kW