Maths Paper 1, November 2024, question 3.1

A circle with radius 20 cm is drawn. A second, smaller circle is drawn so that it passes through the centre of the first circle and touches the first circle from the inside, as shown in the diagram below. A third circle is drawn in the same way: it passes through the centre of the second circle and touches the second circle from the inside. This carries on, so that every new circle has a radius equal to half of the radius of the circle before it. Write down the radius of the 4th circle.