We are going to start with our 50 sided polygon.
Click the nodes button on the left side of the screen.
This will highlight our circle with solid red nodes. At the top of the screen click on the copy button.
On the right side of the screen, under Patterns click on the edit button.
Since both of our circles are the same size we will want to re-size the selected circle. Let's make it larger. Click on the Size button.
To keep our circle looking like a circle and not an oval you will want to lock the aspects so that they change together, so we need to click on the lock icon. We also want to make a dramatic change so let's change our step size to jumbo. Now click the up button.
Notice how the circles are not concentric. We still need to move the highlighted circle. Click the Move button. It is easier to control where we are going if we also change our step to small.
Now use the arrow keeps to move the circle to the desired location.
We now have concentric circles!