Explanation:

To solve this question, we can apply a short trick approach;
If A, B and C can do a work in x, y and z days respectively then all of them working together can finish the work in
(xyz /xy + yz + zx) days
Given:
Time taken by A = x = 12 days
Time taken by B = y = 24 days
Time taken by C = z = 36 days
By the short trick approach:
A, B and C can do the work in
= 12 × 24 × 36 /12×24 + 24×36 + 36×12
After taking 24 as a common term we get,
= 12 x 36 /12 + 36 + 18
After taking 6 as a common term we get,
= 2 x 36 /2+6+3 = 72/11 = 6 6/11