Question 976149
This is familiar.  Check my solutions from up to yesterday and you might find the same question already answered.



r, rate for one worker;
t, unknown number of days for four workers;


RT=J, basic uniform rates equation for doing a task, piece of work, job.  One job is shown in
your example as "1".


{{{system(3r*20=1,4r*t=1)}}}