Question 764402
Find the rates of one man, for your questions #1 and #2.  The question parts will then be simple to answer.


You have the describable form, N laborers do a job in D days.  The rate for one man is then {{{N/N}}} laborers does a job in {{{D*N}}} days.  Think how that works!  The rate for one man in DAYS per JOB would then be {{{1/(D*N)}}}.


Next you want to understand that when workers are working at the same time, then their individual rates are added.  


In both #1 and #2, you will use {{{R*t=j}}} where {{{R}}} is the working rate, {{{t}}} is time, and {{{j}}} is how much or many jobs.  In #1, the unknown variable to solve will be {{{t}}}.  In #2, the unknown variable to solve will be part of the rate ----- a whole number factor representing the rate for the group of unknown quantity of workers.