If each person paints at the same rate, then the amount of work the group accomplishes in any period of time is directly proportional to the number of workers in the group.
If you decrease the number of workers, then the amount of work they do in the same amount of time decreases by the same factor.
If 10 people do one whole job in 5 days, then 70% of that number of people will do 70% of a whole job in 5 days.
