Person A works twice as fast as person B. Person B can complete a piece of work independently in 12 days. Find the number of days taken by person A and B together to finish the work.
If it takes person B 12 days to finish a particular job, that simply means that person A could finish the same job in 12 / 2 = 6 days. So:
1/6 + 1/12 =1/J
J = 4 days for both person A and B working together to finish the job.