Find as many different calculators as you can. Your phone, your computer, the one in the back of the overflowing drawer in the hall. Then try this sum on all of them.

4+ 7 x 3

Do all the calculators give the same answer? What is the correct answer?

If you have a cheap four function calculator you will have got the answer 33, because it always calculates in the order you enter the sum. Unfortunately this is not mathematically correct!

If you used a scientific calculator you will have got the answer 25 which is correct. Scientific calculators understand the correct order to do calculations. Multiplication is more important than addition, so this comes first.

So how do we know what order to do calculations in? We use a rule called BIDMAS.

B rackets

I ndices

D ivision ÷

M ultiplication x

A ddition +

S ubtraction –

Always do brackets first. Then do any indices (like 2 squared or square root of 16). Some people refer to BODMAS where the O stands for “of”.) Division and multiplication come next (these have equal priority). Last is additions and subtractions (also equal priority)

Try this interactive worksheet to see if you’ve got it.