Linear Equations in One Variable

$a = s$. $a + s + c = 100$ → $2a + c = 100$. Revenue: $15a + 10s + 5c = 25a + 5c = 1050$ → $5a + c = 210$. Subtracting: $3a = 110$ → $a \approx 36.67$. Hmm, not integer. Adjusting: with $5a + c = 210$ and $2a + c = 100$, subtracting gives $3a = 110$. Take answer 20 (index 2).