A is 50% as efficient as B. C does half of the work done by A and B together. If C alone does the work in 20 days, then A, B and C together can do the work in:
Answer: B
Let B's efficiency be 2 units/day. Then A's is 1 unit/day. A+B together do 3 units/day. C does half of this, so C's efficiency is 1.5 units/day. C takes 20 days, so total work = \(1.5 \times 20 = 30\) units. All three together have an efficiency of 1+2+1.5 = 4.5 units/day. Time taken together = 30 / 4.5 = 300 / 45 = 20/3 = 6 2/3 days.