All members of our painting team paint at the same rate. If $10$ members can paint a $1000$ square foot wall in $10$ minutes, then how long would it take for $5$ members to paint a $1000$ square foot wall, in minutes?
10 p * 10 min / 1000 ft^2 = 1 p m / 100 ft^2 <==== it takes 1 people min to paint 100 ft^2
Now with 5 people
1000 ft^2 * ( 1 p m / 100 ft^2 ) = 100 p m
then 100 p m / 5 p = 20 min
( if you halve the number of people ....you double the amount of time required)