Answer:
9 days
Step-by-step explanation:
Let x = days taken by C to do it alone
In one day, A can do: 1/12 (part of the whole work)
In one day, B can do: 1/18 (part of the whole work)
In one day, C can do: 1/x (part of the whole work)
In one day,together A, B and C can do: 1/4 (part of the whole work)
we have this equation:
1/x + 1/12 + 1/18 = 1/4
--> 1/x = 1/4 - 1/12 - 1/18 = 1/9
--> x = 9 (days)
