Can anyone explain to me why in expressions you use the PEMDAS theory but then there are expressions you work right to left. Makes no since, how do you differ the two?

Question

anonymous · Answer

Hmm, can you give an example of an expression you work right to left (or left to right)

anonymous · Answer

Yes i sure can:) Okay i take Intens Math on FLVS one of the questions say..


Sarah worked the following problem incorrectly.
Line 1- 40/5(3-1)-4
Line 2- 40/5(2)-4
Line 3- 40/10-4
Line 4- 4-4
Line 5- 0

That was the problem which stated a line was done wrong and to find that line. But when i used PEMDAS i got the same exact answer as she did.

And its not a tick question there is a line that's really wrong and the only way i see that is working the problem from left to right rather than using the PEMDAS theory.

I did say right to left in my original post i meant left to right.

anonymous · Answer

But i dont understand why i wouldn't use PEMDAS in this situation. Like i would every other problem.

anonymous · Answer

is line one$$\frac{ 40 }{ 5 }(3-1)-4~~or~~\frac{ 40 }{ 5(3-1) }-4$$

anonymous · Answer

sorry the / means divide not a fraction
I dont know how to show divide on computer

anonymous · Answer

so its 40 divided by 5(3-1) - 4

anonymous · Answer

oh ok. The first thing you'd do $$40\div5(3-1)-4$$is multiply 5 through, so$$40\div(15-5)-4$$

anonymous · Answer

But the answer would still be 0 even after that. And the problem is saying that the answer isnt 0. Lol idk myself

anonymous · Answer

The only way I can see is if you went like this$$40\div5(3-1)-4$$$$40\div5(2)-4$$$$8(2)-4$$$$16-4$$which equals 12. That's the only way I can see a wrong way of doing it. That satisfies PEMDAS. I think it was written awfully haha. It could have been written much better. You'll always use PEMDAS (and you do in that question), but that question seemed to try to trick you on it by the way it was written. Which is kind of a joke. It's ridiculous when questions are like that.