A's 1 day's work = 1 / 8

B's 1 day's work = 1 / 10

C's 1 day's work = 1 / 20

(A + B + C)' s 1 day's work = \( \Large\frac{1}{8} + \frac{1}{10} + \frac{1}{20} = \frac{5 + 4 + 2}{40} = \frac{11}{40} \)

(A + B + C) can finish the work in 40 / 11 days or \( \Large3\frac{7}{11} \)days