limit equaling zero at infinity I have a problem where I found the percentage of cells as time approaches infinity to be $\frac{100d}{c+d}$. All parameters are positive constants. The question asks are there circumstances when this quantity can be zero? I think if $d$ is small and $c$ is large, the percentage is very close to zero. But if all parameters are positive is it true that the quantity can never actually be zero?

Yes, it is true that if $c,d > 0$, then $\frac{100d}{c+d} > 0$ (so it can't equal $0$).
