Question 173571
Tom can do the job in 7 hours which means he can do {{{1/7}}} of the job in one hour.  Likewise, Johnny can do {{{1/8}}} of the job in one hour.


Together they can do {{{1/7+1/8=8/56+7/56=15/56}}} of the job in one hour.


So they can do the whole job together in {{{56/15}}} hour.


56 divided by 15 = 3 with a remainder of 11.  So 3 hours


11/15 times 60 minutes = 44 minutes


Total time:  3 hours 44 minutes


In general, if A can do a whole job in p time periods and B can do a job in q time periods, together they can do the job in {{{1/(1/p+1/q)=pq/(p+q)}}} time periods.