Question 550575
Person A takes 3 hours to work, so he completes 1/3 work in 1 hr.

Similarly,Person B takes 4 hours to work, so he completes 1/4 work in 1 hr.
 
Working togther they complete 1/3+ 1/4 = 7/12 in 1 hr.

Hence total time taken to complete work = 12/7 hrs (Unitory Method)