If C Alone can Finish the Work in 40 Days :
Answer: (1) 40/3 days
C does half as much as A and B together.
Then, Let (A + B) takes be = X days
Then, C takes to complete same work = 2X days.
Now, that C takes 40 days. (Given in the question)
So 2X = 40.
X = 20
Now, (A + B) takes = 20 days,
And, C takes 40 days,
Then A+B+C takes = 1/20 + 1/40
(2 + 1)/40
Then, A, B and C takes to complete the work = 40/3 days
Hence, the answer is (1) 40/3 days.