The perimeter of a semi-circle is 180 cm. What is its radius?

Let r be the radius of the given semi-circle.

Perimeter of semi-circle = its circumference + its diameter

∴Circumference of a semi-circle = πr = $\frac{22}{7}r$

It is known that the diameter of a semi-circle is equal to 2r.

∴ Perimeter =$\frac{22}{7}r$ + 2r = $\frac{36}{7}r$ 

It is given that the perimeter of the semi-circle is 180 cm.

$$\begin{gathered} \therefore \frac{36}{7}r=180 \\ \Rightarrow r=35 cm \end{gathered}$$

Thus, the radius of the semi-circle is 35 cm.