Precedence

An experienced mathematician would have no difficulty in looking at an expression such as:

3 + 4 x 2 - 1

and assigning it a value of 10. This comes from multiplying 4 and 2, adding 3 then subtracting 1. However, the less experienced might reasonably add 3 to 4 to give 7, and multiply this by 1 (obtained from 2 - 1) to give 7. 

To obtain the correct value one needs to be aware of the precedence rules which govern the order in which we apply the operators with which the numbers are combined within the expression.