Inverses of Power Functions Are Power Functions

The Wi-Fi at school slows down when more people are using their laptops. The Wi-Fi speed can be modeled by the function $f(x)=\frac{1}{2}x^{\frac{1}{2}}+100$ where $x$ is the number of laptops using the Wi-Fi. What is the inverse of this function? How many laptops are using the Wi-Fi if the speed is at $120$