Question 1036631
If it takes u two hours to finish your side that means every hours you complete 1/2 of one side. Similarly ur brother finishes 1/3 of the side every hour. After two hours you have completed ur side and your brother has completed 2-3 of his side and only 1/3 is remained on his side. If you and your brother work together you can complete 5/6 of one side in one hour since 1/2+1/3 = 5/6 
Now we have to see how long takes for both of you to finish only 1/3 of your brothers side. That is 1/3 divided by 5/6 = 6/18 = 1/3 of an hour which is 20 minutes