What is the value of $b$ if $-x^2+bx-5<0$ only when $x\in (-\infty, 1)\cup(5,\infty)$?
When $x<1$ or $x>5$, $-x^2+bx-5<0$. That means that $-x^2+bx-5=0$ at $x=1$ and $x=5$. So, the parabola has roots at 1 and 5, giving us $(x-1)(x-5)=0$. However, we also know the parabola opens downwards since the coefficient of $x^2$ is negative, so we have to negate one of the factors. We can now write $-x^2+bx-5=(1-x)(x-5)=-x^2+6x-5$. Thus, $b=\boxed{6}$.