Given that the equation s = 11h represents the number of stones that two people can place in h hours, then in 3 hours, two people can place 11(3) = 33 stones.
Given that one person can place stones at a rate of 4.5 per hour, then in three hours, one person can place 4.5(3) = 13.5 stones.
Therefore, in 3 hours, 2 people can place 33 - 13.5 = 19.5 more stones than one person.
