Australian AMC 2015 - Senior Question 15

Question 15

Peter was cutting a pipe with an outside diameter of 20 cm. When the cut was just through the wall of the pipe, it was 10 cm in length. How thick was the wall of the pipe in centimetres?

Question illustration

5
$5\sqrt3-5$
$10-5\sqrt2$
$4-\sqrt{10}$
$5(2-\sqrt3)$