Area of a circle is πr^2.
The radius of the smaller circle with diameter 10 cm is 5 cm.
The radius of the larger circle with diameter 20 cm.is 10 cm.
Area of larger circle : Area of smaller circle
Area of larger circle = 100π.
Area of smaller circle = 25π.
Ratio = 100π : 25π or 100π/25π.
