Answer:
C = 42/ 13 hours or 3 3/13 hours
Explanation:
The formula is
1/A +1/B = 1/C
where A is the time for person 1 to do the job alone
B is the time for person B to do the job alone
C is the time for them to do the job together
1/6 + 1/7 = 1/C
The common denominator is 42C so multiply each side by 42C
42 C*(1/6 + 1/7) = 1/C *42C
Distribute
7C +6C = 42
13 C = 42
Divide each side by 13
C = 42/13
Changing to a mixed number
C = 3 3/13 hours