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How do you reduce a 90% gold content into 75%

  1. Sep 23, 2012 #1
    The title says it all.

    Say you have 5 rings, each weighing 18 grams (10% alloy and 90% gold).
    If you would melt them all and add silver to make it 75% gold,

    1) how much silver do you have to add?

    2) Does melting 5 rings with 18grams each give 90 grams in total?

    3) is this equation to solve for this problem right?

    let x = amount of silver to add
    total grams = 90 g

    total grams (.90) + x silver (100%) = total grams + x (.75)
    90(.9) + 1x = 90(.75)
    81 + x = 67.5 + .75x
    x = -54

    4) Am I doing/thinking them all wrong?

    thanks.
     
  2. jcsd
  3. Sep 24, 2012 #2

    Borek

    User Avatar

    Staff: Mentor

    This is a homework type question, I am moving it to HW forum.

    You are right about total mass of gold being 90 g, but this equation

    is wrong (and to be honest I am not even able to decipher what you tried to do).

    Let's assume (like you nicely did), that you added x g of silver.

    If so, what is the formula for the total mass of the alloy?

    What is the formula for the percentage of gold in this alloy?
     
  4. Sep 24, 2012 #3
    more importantly, why do you want to turn more gold to less? That stuff is more expensive than silver anyway
     
  5. Sep 24, 2012 #4
    He is not wanting to turn more into less; he is wanting to turn more into even more.

    Starting with x gram of gold and y gram of other stuff to make (x + y) gram of "gold" (90%), he wants to add z gram of other stuff to make (x + y + z) gram of "gold" (75%).

    And no, even if it is not a homework question, he is not likely to be pulling a swindle. 75% gold is 18 carat gold, which is a much more useful material for certain purposes than pure gold.
     
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