The radius of a circle whose circumference

(c) ∵ Circumference of first circle = 2 πr = πd1 = 36 π cm                                  [given, d1 = 36 cm]

and circumference of second circle = πd2 = 20 π cm                                 [given, d2 = 20 cm]

According to the given condition,

Circumference of circle = Circumference of first circle + Circumference of second circle

$\Rightarrow \quad \pi D=36 \pi+20 \pi \quad$ [where, $D$ is diameter of a circle]

$\Rightarrow \quad D=56 \mathrm{~cm}$

So. diameter of a circle is $56 \mathrm{~cm}$.

$\therefore \quad$ Required radius of circle $=\frac{56}{2}=28 \mathrm{~cm}$