Answer:
101
Step-by-step explanation:
If $\sqrt{100u-100}=100$, then in words, $100u-100$ is the number whose square root is $100$.
The number whose square root is $100$ is $100^2$, that is, $10{,}000$.
(If that's confusing, think of what number you can replace $\textcolor{red}{\heartsuit}$ with so that the equation $\sqrt{\textcolor{red}{\heartsuit}}=100$ is true. It's true that $\sqrt{10{,}000} = 100$.)
So, we have the equation
$$100u-100=10{,}000.$$Adding $100$ to both sides gives
$$100u=10{,}100,$$then dividing both sides by $100$ gives
$$u=\boxed{101}.$$
