A and B can do a work in 12 and 24 days. They work together for 4 days, and then C, who can do it alone in 24 days, joins them. How many more days will the three take to finish it?
Correct answer: B. 3 days
In 4 days A and B do 4 × (1/12 + 1/24) = 4 × 1/8 = 1/2. The remaining 1/2 at 1/12 + 1/24 + 1/24 = 1/6 per day takes 1/2 ÷ 1/6 = 3 days.
hard