Question 1054539
1.{{{F+A+S=300}}}
2.{{{F=S}}}
3.{{{S=50+A}}}
From 3,
{{{A=S-50}}}
Substituting into 1,
{{{(S)+(S-50)+S=300}}}
{{{3S=350}}}
{{{S=350/3}}}
There's a problem with your setup. 
The answer should be an integer, not a fraction.
Check your problem and repost as needed.