- Each of the numbers 1, 2, 3, 4, and 5 appears exactly once in each expression and no other numbers. There is no concatenation or decimal points.

- Each of the four elementary operations (+ - * /) appears exactly once in each expression.

- The only other items which may be in an expression are factorial (!) and parentheses. Factorials and parentheses may occur as many times as needed or not be used at all.

Example: 0=5+3-4*2/1 or 0=(3!+4)/5-2*1