The order of operation rules are:

Rule 1:  Start with calculations inside brackets or parentheses.
Rule 2: Then, solve multiplications and division, from left to right.
Rule 3: Finally, additions and subtraction, from left to right.

 Example 1

Solve 16 + 5 x 8

Based on the rules above, start with multiplication.

This gives: 16 + 40 = 56

Note that if the rule was not followed, and addition was done first, the answer would be different and incorrect, as in,

16 + 5 x 8

21 x 8 = 168 (wrong answer)

Example 2

3 +(35 – 21) x 2

Solve the problem inside the brackets first. Then multiplication, and finally addition.

3 + (35 – 21) x 2
3 + (14) x 2
3 + 28
= 31