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Mixture problem

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Master | Next Rank: 500 Posts
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by towerSpider » Sat Dec 18, 2010 12:21 am
Lily mixed a 45%
copper sulfate solution and a 25%
copper sulfate solution to get 100
mL of a 31% copper sulfate solution.
Haw many mL of the 25% solution
did she use?

My answer comes to seventy but it says that its eighty. Any idea?
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by towerSpider » Sat Dec 18, 2010 12:38 am
Normal method :
---------------
Equations:
Quantity Eq:::: x + y = 100mL
Copper Eq::::: 0.25x+0.55y =
0.31*100
-----------------------------------------
Multiply Quantity by 55
Multiply Copper by 100
---------------------------
55x + 55y = 55*100
25x + 55y = 31*100
---------------------------
30x = 24*100
x = 80mL (amt. of 25% solution
needed in the mix)
==============

Shortcut method of stacey:
25a+45b=c
a=45-31 / 20 = 14/20
b=31-25 / 20 = 6/20
a=100*14/20=70
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by towerSpider » Sat Dec 18, 2010 12:42 am
Sorry guys i got the mistake. It should be 0.45 not 0.55.
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