one group prepared a veggie pizza with a diameter of 20 inches. if the pizza was divided into 8 equal slices what was the area of each slice

The usual formula for the area of a circle is:  A  =  π·r²

Since the diameter is 20 inches, the radius will be 10 inches.

The formula for the total area of the pizza:  A  =  π·10²   --->   A  =  100π

Dividing the area into 8 equal pieces:  100π/8   --->   12.5π  ≈  39.27 sq in