Various representations of Boolean operations

http://en.wikipedia.org/wiki/Boolean_algebra_(logic)

How many of the Irish cows are brown and fat?

Boole proposed that logical propositions should be expressed as algebraic equations. The algebraic manipulation of the symbols in the equations provides a fail-safe method of logical deduction, i.e. logic is reduced to algebra. Boole replaced the operation of multiplication by the word 'and' and addition by the word 'or'. The symbols in the equations can stand for collections of objects (sets) or statements in logic. For example, if x is the set of all brown cows and y is the set of all fat cows, then x+y is the set of all cows that are brown or fat, and xy is the set of all cows that are brown and fat.

Let z = the set of all Irish cows. Then z(x+y) = zx+zy; in other words 'the set of Irish cows that are either brown or fat is the same as the collection of cows that are Irish and brown or Irish and fat'.