Question 1110122
p, pop/rock
t, tropical
u, urban

Done as millions
{{{system(p+t+u=21,p=2(t+u),p=t+9)}}}


Second Equation:
{{{t+u=p/2}}}


Substitute for {{{t+u}}} in the total revenue, First equation:
{{{p+p/2=21}}}

{{{2p+p=42}}}

{{{3p=42}}}

{{{highlight(p=14)}}}

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{{{t=p-9}}}
{{{t=14-9}}}
{{{highlight(t=5)}}}

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{{{2t+2u=p}}}
{{{u=(p-2t)/2}}}

{{{u=(14-2*5)/2}}}

{{{highlight(u=2)}}}


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SUMMARY
p, pop/rock, 14 million
t, tropical, 5  million
u, urban,   2   million