Linear Equations in Two Variables

$30/r + 30/(r+10) = 1$. Try $r = 50$: $30/50 + 30/60 = 0.6 + 0.5 = 1.1$. Try $r = 60$: $0.5 + 30/70 \approx 0.93$. Closer test: $r=50$ gives 1.1, $r=55$ gives $\approx 1.005$. Best of options: $r = 50$.