An old farmer passes away in his sleep. After a suitable period of mourning, his three sons find their father's will in his study.

Amongst other, simpler bequests is the following:

"I wish my small flock of sheep to be well cared for, with the responsibility split between my three suns. Therefore take the pack of cards from my desk drawer, pull out three aces which you will place face up on the desk, then deal out the first three cards from the pack below them to form three fractions. These are the portions into which i wish my flock to be split between my sons."

The sons do as requested, pull out the aces and deal out the three cards, which are a 2, a 3 and a 9, forming fractions one-half, one-third and one-ninth.

Then they go out into the field and count the flock, which turns out to contain 17 sheep.

What cunning sheep-accountancy trick can they use to split the flock according to their father's instructions, without amputating any ovine limbs?