Area and circumference of circles. Game on.

The area of a circle is $A = \pi r^{2}$. Substitute $r = 15$: $A = \pi(15)^{2} = 225\pi\textsf{.}$

The area of a circle is $A = \pi r^{2}$. Substitute $r = 7$: $A = \pi(7)^{2} = 49\pi\textsf{.}$

The radius is half the diameter: $r = \dfrac{16}{2} = 8$. The area is $A = \pi r^{2} = \pi(8)^{2} = 64\pi\textsf{.}$

The area of a circle is $A = \pi r^{2}$. Substitute $r = 9$: $A = \pi(9)^{2} = 81\pi\textsf{.}$

The circumference of a circle is $C = 2\pi r$. Substitute $r = 11$: $C = 2\pi(11) = 22\pi\textsf{.}$

The circumference of a circle is $C = \pi d$. Substitute $d = 20$: $C = \pi(20) = 20\pi\textsf{.}$

The circumference of a circle is $C = 2\pi r$. Substitute $r = 8$: $C = 2\pi(8) = 16\pi\textsf{.}$

The circumference of a circle is $C = \pi d$. Substitute $d = 26$: $C = \pi(26) = 26\pi\textsf{.}$