math question featuring percents

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cdmccrea

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Q: Forty pounds of an alloy of copper and tin contain 8 pounds of copper. How many pounds of tin must be added to make a new alloy that is 10% copper?

My answer is as follows:
40lbs of copper/tin - 8lbs of copper is 32lbs of tin.

(x) lbs of tin = 10% copper (new alloy)

I'm not understanding how to get the answer.
The suggestions are:
40
48
54
60
72

How do I figure the answer for this question?

thank you for your assistance.
 
cdmccrea said:
Q: Forty pounds of an alloy of copper and tin contain 8 pounds of copper. How many pounds of tin must be added to make a new alloy that is 10% copper?

My answer is as follows:
40lbs of copper/tin - 8lbs of copper is 32lbs of tin.

(x) lbs of tin = 10% copper (new alloy)

I'm not understanding how to get the answer.
The suggestions are:
40
48
54
60
72

How do I figure the answer for this question?

thank you for your assistance.
Click to expand...

In the new alloy the amount of copper is 8 lb

If the total amount of new alloy is 'x' then

10% of x = 8

0.1 * x = 8

x = 80

So to make total 80 lbs of alloy - how much more tin you must add?
 
Math is NOT my strong suit.

you asked me a question and I'm baffled about the question and how to come about the answer.

So, I don't know the answer to your question:

by Subhotosh Khan » Mon Nov 16, 2009 7:48 pm
So to make total 80 lbs of alloy - how much more tin you must add?
 
cdmccrea said:
Math is NOT my strong suit.

you asked me a question and I'm baffled about the question and how to come about the answer.

So, I don't know the answer to your question:

by Subhotosh Khan » Mon Nov 16, 2009 7:48 pm
So to make total 80 lbs of alloy - how much more tin you must add?
Click to expand...

This is not math - per se.

You started with 40 lbs of alloy.

You are going to add tin to this alloy to bring up the concentration of tin (think of adding lemon juice in water to make lemonade).

You need to have total 80 lbs of alloy (started with 40 lbs of alloy) - how much (weight) tin do you need to add to get to the final weight?

If really cannot decipher that - you need a face-to-face tutor.
 
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