If three circular metal discs of equal thickness whose radii are 5cm, 20cm and 40cm are melted down to form one circular disc of the same thickness, find its radius.

45 (cm)

Step-by-step explanation:

Since the discs have the same thickness, we will think about the areas.

Let say x the new radius:

ca = ec

cpctc makes it so that corresponding parts of the triangle you proved to be equal are equal, the other options aren't part of the part of the triangles you proved to be equal.