Question 745429
Volumes of compound A and B assign to x and y.

Description with those variable assignments renders {{{x/y=5/3}}} and {{{x+y=664}}}.  Just solve the system for x and y.  


The question asks specifically, how much of compound A, assigned to variable x.  You can solve one of the equations for y and substitute this into the other equation and solve for x.  That will be your solution for x.  


How about this:
{{{x+y=664}}}
{{{y=664-x}}}
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{{{x/y=5/3}}}
{{{x=5y/3}}}
{{{x=5(664-x)/3}}} according to the substitution for y
{{{x=(5*664-5*x)/3}}}
{{{3*x=5*664-5*x}}}
{{{3x+5x=5*664}}}
{{{8x=5*664}}}
{{{x=5*83}}}
{{{highlight(x=415)}}}, milliliters of compound A