PUZZLE 091 Circular Reasoning

We have here a semi-circular piece of paper.
It's a circle with a circumference of 30cm that has been cut in half.
Using this piece of paper, what is the circumference of the biggest possible circle you can make?
You can fold or bend the paper, but you cannot cut it.

Hint 1

It would be hard to make a flat circle.
You need to think about this in 3D.

Hint 2

Try making the semi-circle into a cone.
Take the middle of the semi-circle as the upper point and fold the paper around to make a cone.

Hint 3

If you bend the paper to make a cone, the bottom of the cone will form a circle.
The circumference of the circle will be the same as the length of the arc of the semi-circle.

Super Hint

Continuing on from hint three...
The cone is made from a piece of paper that is half of a circle with a circumference of 30cm.
So what would be the circumference of the bottom of the cone?

Answer for 'Circular Reasoning'

The answer is '15'.