They could do this with a line and with a parabola. Use two functions. You have to change m in one, and b in the other. Make them intercect within the zoom, make them not intercect within the zoom. A child should be able to state an answer and support it with a full explanation.
Mentors have to work harder when questions lead to multiple answers.
In a sense all of our problems are open-ended. Choosing problems for collaborative teams to elaborate. Students can change number of wedges, color of wedges, and location of wedges. Perhaps students can define their own functions using a spread sheet or data table. Perhaps use chores in a family instead of raffle.