Question 889347
Jims copier does 1/7 of a job per hour
Janets does 1/12 per hour

Set t = time to complete a job
t(1/7) + t(1/12) = 1 job
t(1/7 + 1/12) = 1
use 7*12 as a common denominator
{{{t( ((12)/(12))*(1/7) + ((7)/(7))*(1/12) ) = 1}}}
{{{t ( 12/(12*7) + 7/(12*7) ) = 1 }}}
{{{t ( 19/84 ) = 1 }}}
multiply each side by {{{84/19}}}
t = {{{84/19}}} = 4.421 hours