Question 993451
The plumber can replace 1/x of the pipes in 1 hour. (so x hours to replace by himself)
The apprentice can replace 1/4x of the pipes in 1 hour. (so 4x hours to replace by himself)


1/x + 1/(4x) = 1/14
5/(4x) = 1/14
x = 35/2


The plumber takes x = 35/2 hours to replace the pipes alone.
The apprentice takes 4x = 70 hours to replace the pipes alone.