1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 = 10
Replace the 'x' by '+', '-', '*', '/'.
No operator occurs more than twice.
Evaluation is left to right, no brackets, e.g. 1+2*3-4= 3*3-4=5

Source: Zeit Magazin, Nr 13, 22. Mar. 96