This is math problem that requires the scientific method of inquiry. I would not give the answers, but I might allow a week or more and repeat the questions a few times until students come up with theories.
Each clay body shrinks at a slightly different rate.
Ask the students: How can we predict the exact shrinkage for any piece of soft clay so that we could make a lid for a piece that needs a replacement lid that fits correctly?
How can we account for some clay being softer (containing more water) than other pieces of soft clay as they work with it? What if you work with clay that is harder or softer? See water of plasticity (WOP) message.
Can we figure out formulas that account for both shrinkage and for different softness of clay?
Can they make shrinkage rulers, fire them and derive formulas from them to help predict the finished fired size of a clay piece?