Explanation:
pizza = circle
the area of a circle is
pi×r²
r = radius (half of diameter), in our case 15/2 = 7.5 in.
the total area of the whole pizza (circle) is
pi×7.5² = 56.25×pi = 176.7145868... in²
it is cut into 12 slices. Carson eats 5 of these 12 slices.
that means he eats 5/12 of the whole area :
176.7145868... × 5/12 = 73.63107782... in²