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Math Help - Simple PEMDAS ? - TIA

  1. #1
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    Simple PEMDAS ? - TIA

    Hi all,

    Been a while since I went through my Order of Operations and I have a problem that I know the answer to but I need someone to help me see why the answer is correct... if that makes sense.

    Problem is 3-4[-10-4(-5)]
    Correct Answer is supposed to be: -37

    1) I know to work inside parens first...
    2) Since there is no real work inside the Parenthesis... this is where I mess up I think... I would do Multiplication next... so I would take the -4 * -5 and get +20... correct?
    3) Next I would then take the -10 * +20... I know this can't be right.. so I try again... using only the +4 * (-5) and that gets me closer but doesn't really make sense to me because the -10 - 4 is actually a negative four right? Arrggh!
    4) So then -10 * -20 doesn't work either... please help!

    Step by step is ok... but really need the Why. Thanks in advance for your assistance and advice.
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  2. #2
    A riddle wrapped in an enigma
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    Quote Originally Posted by garms View Post
    Hi all,

    Been a while since I went through my Order of Operations and I have a problem that I know the answer to but I need someone to help me see why the answer is correct... if that makes sense.

    Problem is 3-4[-10-4(-5)]
    Correct Answer is supposed to be: -37

    1) I know to work inside parens first...
    2) Since there is no real work inside the Parenthesis... this is where I mess up I think... I would do Multiplication next... so I would take the -4 * -5 and get +20... correct?
    3) Next I would then take the -10 * +20... I know this can't be right.. so I try again... using only the +4 * (-5) and that gets me closer but doesn't really make sense to me because the -10 - 4 is actually a negative four right? Arrggh!
    4) So then -10 * -20 doesn't work either... please help!

    Step by step is ok... but really need the Why. Thanks in advance for your assistance and advice.
    Hi garms,

    Let's take this step by step:

    3-4[-10{\color{red}-4(-5)}]

    Do the operation in red first:

    3-4[-10{\color{red}+20}]

    Next, simplify what's within the brackets:

    3-4[+10]

    Next, multiply by -4:

    3-40

    And you get your desired result.
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  3. #3
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    Thanks Masters!

    Thank you, that makes perfect sense now!! As e^(i*pi) pointed out I was making a simple error in step 3 but the -40 that you showed me was getting me also in one of my many attempts of reworking this various ways. You guys Rock!!

    This stuff, for me is kinda like that e-mail that you get where most of the vowels are removed yet your brain can still read it just fine... except.. not words... and my brain sees the wrong stuff! lol :-) thanks for the clarification. Step by step really helped me see (red was nice touch)

    best regards,

    GA
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