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?

We have that
$$\frac{100d}{c+d}=0 \iff d=0$$
and
$$\lim_{d/c\to 0} \frac{100d}{c+d}=0$$
$$\lim_{c/d\to \infty} \frac{100d}{c+d}=0$$
and you are right since all quantities are positive the ratio can't never be zero.
