If you add 10,30 and 20 you get 60 % in total meaning that 3600 men is equivalent to 40%. Now you know this, if you find whatever 3600 is 40% of, you can work out the answer. You can do this by finding 50% of 3600 which is 1800, which is 20% of the total amount, then doing 3600+3600+1800 to find 100% (40+40+20%) which gives you an answer of 9000.
