A and C, B and C and A, B and C can complete a work in 20, 12 and 8 days respectively. In how many days A and B together can complete that work?

Let the total work be 120 units (LCM of 20, 12 and 8)
Efficiency of A+C = 120/20 = 6 units/day
Efficiency of B+C = 120/12 = 10 units/day
Efficiency of A+B+C = 120/8 = 15 units/day
Then, Efficiency of A = 15-10 = 5 units/day
Efficiency of B = 15-6 = 9 units/day
Efficiency of C = 15-(5+9) = 15-14 = 1 unit/day
Total efficiency of A and B = 14 units/day
Therefore, A and B together can do the work in $$\dfrac{120}{14} = \dfrac{60}{7}$$ days