If it take john 10hours to paint the fence, how long will it take john and his friends to do the job if they work at the same rate

If it takes John 10 hours to paint the fence, then it takes him
(10/2) = 5 hours to paint each half of it.

If John's friend is willing to help him, and the friend works at the
same rate that John works, then they can split the job into its
two halves, and each man can finish his half in 5 hours.

If John has more than one friend, and they ALL want to work
on the job together, and they ALL work at the same rate, then
they should split up the job into equal parts, and they'll finish
in

         (10)  divided by  (1 plus the number of friends)         hours.
 

Iam guessing c. or d.

5/ 1/3=15 i think sorry if im wrong