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Twice as many? 3/4 as many? How to interpret?

  1. Nov 14, 2011 #1


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    How do you write "twice as many x as z" algebraically?

    Is this 2x=z or x=2z? How do you know?

    How about "3/4 as many x as z" algebraically?

    Is this (3/4)x=z or z=(3/4)x? How do you know?

    An applied example:
    There are twice as many apples as there are oranges, so does this mean for each orange there are 2 apples, so that if apples=x and oranges=y, then y=2x; 2y=4x, and etc?

    Are these statements ambiguous to you?
    Last edited: Nov 14, 2011
  2. jcsd
  3. Nov 14, 2011 #2
    x = 2z

    There's no trick to it; all of the information is right there in the expression. It says that you need 2 of z to equal 1 of x, hence: x = 2z.
  4. Nov 14, 2011 #3


    Staff: Mentor

    x = 2z

    An equivalent problem is "x is twice as big as z." It might help to look at numbers. 50 is twice as big as 25. Would you write an equation expressing this relationship as
    a) 50 = 2*25
    b)2*50 = 25?
    x = (3/4)y
    I would not let apples = x and oranges = y. These variables should represent the number of apples or oranges.

    As far as translating to equations, see what I said at the beginning of this post.
  5. Nov 14, 2011 #4


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    I don't know what has gotten to me, sometimes I just can't get something. It must be doing my math homework for 6 hours straight... I can't think straight anymore. Anyone else have similar experience? Or it must be something I ate...
  6. Nov 16, 2011 #5
    I usually try to think of this in terms of ratios. The phrase "twice as many x as z" gives you an equality of ratios, x:z is 2:1. Assuming z isn't zero, we can rewrite this using fractions as [itex]\frac{x}{z} = 2 [/itex] so x = 2z.
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