We should think of it this way: for a ***60-hour workload, the first part of the work was done by hand for (5+4=9) hours, and then three people were added for 4 hours, so the people added later did (3*4= 12) hours of work, leaving (60-12).

Solution: suppose you want to arrange x people to work.

(x-3)*(5+4)+3*4=60

X=8 and 1/3.

Because the number of people is an integer, nine people should be arranged.