Sally is selling delicious cupcakes. As a result of paying
attention to changes in cupcake demand for price changes to
identify demand for cupcakes. she found that cupcake sales
fluctuate from 30 to 24 if the unit price of cupcakes changes from
4 to 5. Now, derive the linear demand curve of the cupcake, and
obtain the price level of the cupcake whose price elasticity of
demand is -1.