SOLUTION: I am truly stumped with this one. An alloy weighing 20 kg is 30% copper. How many kg of another alloy, which is 80% copper, must be added in order to produce an alloy which is

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Question 11749: I am truly stumped with this one.
An alloy weighing 20 kg is 30% copper. How many kg of another alloy, which is 80% copper, must be added in order to produce an alloy which is to be 50% copper?

Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
Ok, here we go!
First change percentages to decimals:
30% = 0.3
80% = 0.8
50% = 0.5
Now you can set up the equation in which x is the number of Kg. of 80% Cu alloy needed.
20%280.3%29+%2B+x%280.8%29+=+%2820+%2B+x%29%280.5%29 Simplify and solve for x.
6+%2B+0.8x+=+10+%2B+0.5x Subtract 0.5x from both sides.
6+%2B+0.3x+=+10 Subtract 6 from both sides.
0.3x+=+4 Now divide both sides by 0.3
x+=+13.33
13.33... Kg of 80% copper alloy are needed.