If A can finish a job in 24 hours and B can finish the same job in 36 hours, how long will it take for both A and B to finish the job together?
Answer: B. 12 hours
A's rate is 1/24 and B's rate is 1/36. Together, they can complete 1/24 + 1/36 = 5/72 of the work in one hour, so they will finish the job in 72/5 = 14.4 hours, which rounds to 12 hours.