Answer:
9 days
Step-by-step explanation:
A = 1/12 = .083333 completed each day
B = 1/18 = .055555 completed each day
C = 1/x completed each day
Finished piece 1/4 = .25 completed each day
A + B + C = .25
.08333 + .05555 + C = .25
.138888 + C = .25
C = .25 - .138888
C = .11111
C = 1/x
.1111111 = 1/x
.1111111x = 1
x = 9
It will take C 9 days to finish a piece of work alone.