Question 970459
I will say that the quantity of metal that I
start out with is 100 units ( the units don't 
matter -I can make them up )
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That means I start with 90 units of bronze,
7 units of gold, and 3 units of silver
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Now I will say there are {{{ a }}} units of the 
2nd mixture where {{{ a }}} could be any number
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In the 2nd mixture, there is:
{{{ .85a }}} units of bronze
{{{ .05a }}} units pf gold
{{{ .1a }}} units of silver
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The amount of gold did not change, so
{{{ .05a = 7 }}}
{{{ a = 140 }}} units of final mixture
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{{{ .85a = .85*140 }}}
{{{ .85a = 119 }}} units of bronze
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{{{ .1a = .1*140 }}}
{{{ .1a = 14 }}} units of silver
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I started with 90 units of bronze, and
ended up with 119, so there were 
{{{ 119 - 90 = 29 }}} units of bronze
in the 2nd mixture
{{{ 140 - 100 = 40 }}} units in 2nd mixture
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{{{ 29 / 40 = .725 }}}
There is 72.5% bronze in the 2nd mixture
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check the answer:
Silver:
{{{ .1*140 = 14 }}} 
{{{ 14 - 3 = 11 }}}
{{{ 11/40 = .275 }}}
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{{{ .725 + .275 = 1 }}}
correct, because no additional gold was added
Hope you can follow this and it makes sense