Divide the field into just eleven enclosures by means of
straight fences, so that every enclosure shall contain one tree as a
shelter for cattle.

How do you do it with as few fences as
possible?

Take your pencil and draw straight lines across the field
until you have marked off the eleven enclosures (and no more), and then
see how many fences you require.

Of course the fences may cross one
another.