Three Fruits Poster

Answer: Quince, quonce, quance


7 quince weigh the same as 4 quance, so a quince weighs less than a quance. [1]

5 quance weigh the same as 6 quonce, so a quonce weighs less than a quance. [2]

So quance are the heaviest.

20 quance must weigh the same as 35 quince (from [1])
20 quance must weigh the same as 24 quonce (from [2])

So 35 quince weigh the same as 24 quonce, so a quonce weighs more than a quince.

So quince is the lightest, and quance is the heaviest.