Question 1039506
Let's put 
chairs: c
tables: t

{{{3*c+1*t=900}}}
 so 
{{{t=900-3*c}}}
{{{5*c+3*t=2100}}}
By substituting t from the above equation, we get:
{{{5*c+3*(900-3*c)=2100}}}
{{{5*c+2700-9*c=2100}}}
{{{2700-2100=9*c-5*c}}}
{{{600=4*c}}}
{{{c=150}}}

So 
{{{t=900-3*150=450}}}

{{{4*c+t=4*150+450=1050}}}