All of the members of our spray-painting team paint at the same speed. If $5$ team members can paint a certain wall in $27$ minutes, then how many minutes would it take $15$ of our team members to paint the same wall?
It would take 5*15/27 = 25/9 minutes.
An increase of 15 / 5 ==3x in manpower will entail a decrease in time by the same factor:
That is: 27 minutes / 3==9 minutes.
OR:
[5 x 27] / 15 ==9 minutes.
