Let's solve this step by step using the work per day method.
If A and B together finish the work in 12 days, then their 1-day work is:
(A + B)=1/12
If A, B, and C together finish the same work in 8 days, then their 1-day work is:
(A + B + C)=1/8
Now subtract the 1-day work of A + B from A + B + C:
C’s 1-day work=1/8-1/12
To subtract, find LCM of 8 and 12 = 24:
1/8=3/24 &1/12=2/24
3/24-2/24=1/24
If C does 1/24 of the work per day, then C alone will finish the job in:
1/1/24=24(Hint a/b/c=ac/b)