I live in city $B$. Assume that i'm going on a vacation to city $A$. Before i go there, for every night i should pay 60 dollars and after i go there, for every other night i should pay 100 dollars.
After i go there, the company i work for, might call me for something important to return to city $B$. The probability that this happens in $0.3$ for every day.
Question : How many days it's better for me to pay for ? (before going to the trip)
Note : I have no idea about this question and i don't know how that probability is related to the question.
Thanks in advance.