Leon works at the aquarium. When he tries to put each turtle in its own tank, he has one turtle too many. But if he puts two turtles per tank, he has on tank too many.
How many turtles and how many tanks does Leon have?
Leon has 4 turtles and 3 tanks.
When Leon puts 1 turtle in each tank, he has an extra turtle. When Leon puts 2 turtles in each tank, he has an extra tank.