Lisa and Alan have to calculate the area of each Pizza, and then calculate the unit area of Pizza per Dolla of each size, then they finally decide which one gives more benefit.

Size of pizza is diameter, the radius is a half of diameter, they are 8 inch and 5 inch respectively.

Area of the pizza 16 inch:

A1 = 8*8*3.14 = 200.96 (inch^2)

Unit/1$ = A1/price = 200.96/11.99 = 16.76 (inch^2/$)

Area of the smaller

A2 = 5*5*3.14 = 78.5 (inch^2)

Unit/1$ = A2/price = 78.5/5.99 = 13.10 (inch^2/$)

This mean Lisa and Alan should buy the bigger one.