All members of our painting team paint at the same rate. If $10$ members can paint a $100$ square foot wall in $10$ minutes, then how long would it take the $15$ members to paint a $100$ square foot wall, in minutes?
Each member paints 100 / (10 * 10) = 1 sq ft per minute
So 15 will paint 15 sq ft in a minute
So 15 will take
100 / 15 = 20 / 3 = 6 + 2/3 min = 6 min 40sec
+721 
