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Math Help - Simple algerbra problem

  1. #1
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    Simple algerbra problem

    Hi everyone,
    i know i am way out of my league being on this forum but i am a mature student looking to get into college & for the corse i want i ned to repeat my maths exams! I tried signing up to a class but the pace was too fast for me. Anyway any help with this simple problem would be appreciated, thanks in advance!
    Find the missing number in the following calculation:
    -5x3x?=30
    My question is this,
    what is the formula or procedure to finding out the missing number, other examples are
    (7?-5)x3=-9
    or
    ?-(-3)x(-2)=-4
    thanks again and i do apoligise for asking such a stupid question on here!
    Laura
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  2. #2
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    Re: Simple algerbra problem

    You can study this guide.
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  3. #3
    MHF Contributor MarkFL's Avatar
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    Re: Simple algerbra problem

    Traditionally in algebra, we represent the unknown quantity with a letter such as x or y, but any symbol will do, such as a question mark.

    In the first example, we are given:

    -5\times3\times?=30

    The associative and commutative properties of multiplication say that we may move/group factors as we choose, .i.e:

    a\times b\times c=(a\times b)\times c=a\times(b\times c)=a\times c\times b, etc.

    So, we may write the equation as:

    (-5\times3)\times?=30

    Now, compute the product within the parentheses:

    -15\times?=30

    Since the unknown quantity is being multiplied by -15, we want to divide both sides by -15 to isolate the unknown on its side of the equation. Whatever we do to one side of the equation we must also do to the other.

    \frac{-15\times?}{-15}=\frac{30}{-15}=\frac{(-2)\times(-15)}{-15}

    Now, we may cancel or divide out factors common to the numerators and denominators, and we are left with:

    ?=-2

    Second example: (7?-5)\times3=-9

    Divide both sides by 3:

    \frac{(7?-5)\times3}{3}=\frac{-9}{3}=\frac{-3\times3}{3}

    7?-5=-3

    Now we want to add 5 to both sides to isolate the term with the unknown factor ( x+0=x):

    7?-5+5=-3+5

    Simplify:

    7?=2

    Since the unknown is not separated by any characters, it is implied that we have the product 7\times?.

    Divide both sides by 7:

    \frac{7?}{7}=\frac{2}{7}

    ?=\frac{2}{7}

    Third example: ?-(-3)\times(-2)=-4

    Simplify by computing the product on the left side (a negative times a negative is positive, but this positive is being subtracted):

    ?-6=-4

    Add 6 to both sides:

    ?-6+6=-4+6

    Add like terms:

    ?=2
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  4. #4
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    Re: Simple algerbra problem

    -5x3x?=30
    In this question we find out the ? as:
    ?=30/(-5*3)
    ?=30/(-15)
    ?=-2

    Now similarly, all the other questions of this category can be solved same:
    (7?-5)x3=-9
    (7?-5)=-9/3
    (7?-5)=-3
    (7?=-3+5
    7?=2
    ?=2/7

    Similarly,
    ?-(-3)x(-2)=-4
    ?-(-3)=-4/-2
    ?-(-3)=2
    ?=2/3

    Thus you can see you can solve every question falling under this category is solved step by step & equalizing each term accordingly.
    Last edited by skeeter; September 21st 2012 at 05:13 AM. Reason: removed commercial link
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  5. #5
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    Re: Simple algerbra problem

    Thanks so much for all of your replies I really appreciate it! I might be back for more help soon ( well might is a lie I will be back soon ;-)
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