To start, you must understand what radius and diameter is. Diameter is the full length of something, like the full length across a circle, and radius is simply just half of the diameter. So if you’re given the diameter and you need to find the radius, divide the diameter by 2. If you’re given the radius and need to find the diameter, multiply the radius by 2.
So let’s start:
Radius - Diameter
6cm - 12cm
10cm - 20cm
70cm - 140cm
125cm - 250cm
2.5cm - 5cm
1.8cm - 3.6cm
Hope this helped!