(A + B)'s 1 hour's work = \(\left(\frac{1}{12}+\frac{1}{15}\right)=\frac{9}{60}=\frac{3}{20}\)

A + C)'s hour's work = \(\left(\frac{1}{12}+\frac{1}{20}\right)=\frac{8}{60}=\frac{2}{15}\)

Part filled in 2 hrs =\(\left(\frac{3}{20}+\frac{2}{15}\right)=\frac{17}{60}\)

Part filled in 6 hrs = \(\left(3\times\frac{17}{60}\right) =\frac{17}{20}\)

Remaining part = \(\left(1-\frac{17}{20}\right) =\frac{3}{20}\)

Now, it is the turn of A and B and \(\frac{3}{20}\)  part is filled by A and B in 1 hour