Question 149079
If the plumber can do the installation in 50 minutes, then he can do {{{1/50}}} of the installation in 1 minute.  Likewise, working together, the plumber and his apprentice can do {{{1/40}}} of the job in a minute.  We need to find out how many minutes it takes the apprentice by himself and we'll call that {{{x}}}, so the apprentice by himself can do {{{1/x}}} of the job in one minute.  So we can write:


{{{1/50+1/x=1/40}}}


{{{1/x=1/40-1/50}}}


LCD for 40 and 50 would be 200, so:


{{{1/x=5/200-4/200}}}


{{{1/x=1/200}}}


{{{x=200}}}


So it would take the apprentice 200 minutes or 3 hours and 20 minutes to do the installation by himself (presuming, of course, that the apprentice could do it by himself at all).