When a calculation mixes operations, the order is fixed and it is not left to right. Work out anything inside **brackets** first. Then do all the **multiplication and division**, taking those left to right. Only then do the **addition and subtraction**, again left to right. Left to right settles a tie between two operations of the same rank — it never lets a plus jump ahead of a times. The same rule runs the multi-step word problems: 'bought this many, gave some away, shared the rest equally' is one number sentence with brackets around the part that has to happen first.
Worked example
Problem Work out 18 + 6 × (9 − 3).
Brackets first: 9 − 3 = 6, so the expression becomes 18 + 6 × 6.
Multiplication outranks addition, so do it next: 6 × 6 = 36.
Add last: 18 + 36 = 54, so the answer is 54.
Watch out Reading left to right and doing 18 + 6 first gives 24 × 9 − 3 = 213 — a completely different number. The order of operations is not optional.