SOLUTION: 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?

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Question 435711: 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?

Answer by rwm(914) About Me  (Show Source):
You can put this solution on YOUR website!
Right now the alloy has 8/40=1/5 copper
so we want half of that 1/10 to be copper.
The temptation is to double the tin amount to 64 instead of the present 32.
but
We double the total amount not just the tin.

1/10=8/80
80-40=40
so we add 40 lbs of tin to have a total of 80 lbs.
we could write it
.9a+.1a=40+x
.1a=8
where a=80 and x=40