Question 860560
By "similar" you really mean, "congruent", so same size and shape.


The rate for 1 worker in the first described group is 1 "paint the building" in {{{20*8=160}}} days.


For a group of n workers, the work rate of this group is {{{n(1/160)}}} jobs per day.  The basic idea of uniform rates for task completion or doing a job is R*t=j for Rate, time, job.  Your question simply becomes {{{highlight(highlight((n/160)15=1))}}}, using units of days and job.  n is the count of the workers needed in the group, and unknown which easily is solvable.