Each number is used once; no repetition is allowed.
The calculations are done from left to right (i.e., 7-2x3=15, not 1) and may involve only positive numbers (so 5-3-7=-5 is not allowed).
_ _ _ _ _ _ _ _ _ | | - | | x | | = | | | | x | | x | | = | | + | | + |_| + |_| + | | + |_| |_| - |_| x |_| = |_| - |_| - |_|