To use instruments in a shared research lab you first have to ask the lab monitor. She will then walk into the lab to see if the instrument you want to use is available, and if it is, you get to use it. One popular instrument is in use 81% of the time.
a. Define X as the number of times you request to use the instrument and you are successful.
Assuming each of your n requests is an independent event, what is the pmf for X?