Question 1038701
n nuts and r raisins;
{{{n+r=120}}}
{{{n=120-r}}}
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Or you can simply use r for how much raisins, and then 120-r is how much nuts.  This then is using only a single variable, r.
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Cost must be {{{120*8}}} dollars.


{{{12n+6r=120*8}}}
{{{12(120-r)+6r=120*8}}}
Quick simplification,
{{{2(120-r)+r=20*8}}}
{{{240-2r+r=20*8}}}
{{{240-r=20*8}}}
{{{r=240-160}}}
{{{highlight(r=80)}}}
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{{{highlight(n=40)}}}


The method using another example is also shown here:
<a href="https://www.youtube.com/watch?v=-REjxT9KCzQ">just one variable for two-part mixture video example</a>