Shamus can paint the barn in 6 hours, and ishmael can paint the barn in 7 hours. how long will it take them to paint the barn together?

Given that Shamus can paint the barn in 6 hours, the portion of barn he can complete in 1 hour will be 1/6
If Ishmael can paint the ban in 7 hours, the portion he can paint is 1/7
when working together the portion they can complete will be:
1/7+1/6
=13/42
time taken for them to complete the barn will be:
42/13
=3 3/13 hours
the answer is 3 3/13 hours