On Arbor Day the fourth grade class began planting trees. They finished planting five trees before the fifth grade class arrived. But they accidentally planted them on the fifth grade side of the street.
The fourth-graders crossed the street to start over, and the fifth-graders planted the remaining trees. They finished first and felt bad for the fourth-graders, so they crossed the street and planted five trees. They planted another five trees at which point all of the trees had been planted.
By how many trees were the fifth-graders ahead of the fourth-graders?