Is there a rule in mathematics that states that expressions shall be evaluated from left to right assuming that all operators in the expression share the same precedence? 

e.g. given the expression `6 / 2 * (1 + 2)`  

is there a rule so that only `(6 / 2) * (1 + 2)` (result: 9) is a valid representation and  

`6 / (2 * (1 + 2))` (result: 1) is invalid? 

**edit**: I am asking this, because I get different results on different calculators.