Probability of Bosons in Ideal Gas: Finding Average Number

[shirashi]

Homework Statement

An ideal Bose gas is confined inside a container that is connected to a particle reservoir. Each particle can occupy a discrete set of single-particle quantum states. If the probability that a particular quantum state is unoccupied is 0.1 then what is the average number of bosons in that state?



2. The attempt at a solution
How do i solve this problem? which formula should be employed? I think if its connected to a particle reservoir then all the energy levels should be filled and nothing should remain empty.