Let t be the total time A, B and C can do a piece of work

Thus, A can do that work in t + 6, B in t +1, and C in 2t

Working together: \(\frac{1}{T} = \frac{1}{T+6} + \frac{1}{T+1} + \frac{1}{2T}\)

\(\frac {(t + 6)(t + 1)(2t)}{x} = (x+1)(2t) + (x+6)(2t) +(x+6)(c+1)\)

\(2x^2 +14x +12 =5x^2 +21x + 6\)

\(3x^2 +7x -6 = 0\)

\((3x-2)(x+3) = 0\)

\(x = 2/3\) hours

\(x = 2/3 (60) \) = **40 minutes**