Question 278485
Since they both equal P, set them equal to each other, and solve for Q.
{{{P=1600-20Q}}}

{{{P=100+10Q}}}
{{{1600-20Q=100+10Q}}}
{{{-30Q=-1500}}}
{{{Q=50}}}
Then use either equation to solve for P. 
{{{P=1600-20Q}}}
{{{P=1600-20(50)}}}
{{{P=1600-1000}}}
{{{P=600}}}