66. Two men have an equal interest in a grindstone, which is 5 ft.
6 in. in diameter. The centre of the stone, to the extent of a
diameter of 18 in., is useless, and not to be taken into account.
Required to find the depth to which the first partner may be allowed
to grind away from the stone in order to leave an equal share of the
stone to the second partner.
[Illustration]