10 people can paint a building in 5 days. if each person paints as quickly as the others then how much of the building could 7 people paint in 5 days?

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.