Question 171390
let the number of tickets sold to non seniors and seniors , be n and s  respectively
:
n+s=400.........eq 1
7n+5s=2600......eq 2
:
lets rewrite eq 1 to s=400-n and place that value into eq 2.
:
{{{7n+5(400-n)=2600}}}
:
{{{7n+2000-5n=2600}}} distribution
:
{{{2n=600}}}combining terms
:
{{{highlight(n=300)}}}number of non senior ticket sold