11 Plus Exams Forum

Using BIDMAS I get an answer of 2 for the following question. The actual answer is 8. Can some one explain why? Should I not be using BIDMAS or I am somehow using it incorrectly? Thank you

25-5x4+3

Using bidmas you would do the multiplication first to give you 25-20+3=8

Tinkers wrote:Using bidmas you would do the multiplication first to give you 25-20+3=8

But then shouldn't you do addition before subtraction which would give you 25-20+3 and then 25-23=2. I know that is wrong but still not sure why?

Hera wrote:But then shouldn't you do addition before subtraction which would give you 25-20+3 and then 25-23=2. I know that is wrong but still not sure why?

You can't 'divorce' the sign and the number so doing addition first gives you 25 + 3 = 28 then 28 - 20 = 8

Thank you. Got it now

Hera wrote:

Tinkers wrote:Using bidmas you would do the multiplication first to give you 25-20+3=8

But then shouldn't you do addition before subtraction which would give you 25-20+3 and then 25-23=2. I know that is wrong but still not sure why?

Also, -20+3 will never give you -23. It is the same as +3-20, so -17. You would only get -23 if there was a bracket in there, in which case you would do the addition in the bracket first, i.e. -(20+3) = -(23) = -23

I think this query highlights quite a common misunderstanding of BIDMAS which is due to taking the acronym too literally.

Perhaps the simplest way to get it right is to be aware that, despite what the BIDMAS acronym seems to imply, there is no real ordering between "addition and subtraction". There is also no ordering between "division and multiplication".

So my advice would be if given a sum containing just addition and subtraction (or just multiplication and division) forget BIDMAS and just simply work it out left to right.

Proud_Dad wrote: be aware that, despite what the BIDMAS acronym seems to imply, there is no real ordering between "addition and subtraction". There is also no ordering between "division and multiplication".

So my advice would be if given a sum containing just addition and subtraction (or just multiplication and division) forget BIDMAS and just simply work it out left to right.

important point!

I disagree - sometimes there is a 'most efficient way of calculating:

25 + 7 - 5 is best done in the order 25 - 5 + 7 which is quicker than tackling in order.

Guest55 wrote:I disagree - sometimes there is a 'most efficient way of calculating:

25 + 7 - 5 is best done in the order 25 - 5 + 7 which is quicker than tackling in order.

Very true - that makes a much easier calculation. It relies, though, on a person remembering your original point - that a sign cannot be 'divorced' from a number. It took quite a while for my youngest to learn that the + or - sign belong to the numbers in front of which they are written, and stop ignoring the signs while re-ordering numbers in calculations.