Ask Uncle Colin: Oh God, Not This Again

Dear Uncle Colin,

When evaluating an expression, after you've dealt with brackets, powers and roots, do you deal with multiplication and division left to right, or does multiplication take precedence?

– BIDMAS Idiocy Doesn't Make Any Sense

Hi, BIDMAS, and thanks for your message!

There are three answers to this.

If you're a computer, you evaluate operators of the same precedence – such as multiplication and divide – from left to right, by convention.

If you're a human, you try to figure out from the context what the person writing the expression meant, regardless of BIDMAS. Someone on Reddit might write 1 / x-3 to mean $\frac{1}{x-3}$ rather than the $\frac{1}{x} – 3$ a strict interpretation of the rules gives.

And if the question is $6 \div 2(1+2)$, you say “This is fake maths, write the bloody thing properly.”

Hope that helps!

– Uncle Colin


Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008.

He lives with an espresso pot and nothing to prove.


Leave a Reply

Your email address will not be published. Required fields are marked *

Sign up for the Sum Comfort newsletter and get a free e-book of mathematical quotations.

No spam ever, obviously.

Where do you teach?

I teach in my home in Abbotsbury Road, Weymouth.

It's a 15-minute walk from Weymouth station, and it's on bus routes 3, 8 and X53. On-road parking is available nearby.

On twitter