An experiment has 10 equally likely outcomes. Let $A$ and $B$ be two non-empty events of that experiment. If $A$ consists of $4$ outcomes, find the number that $B$ must have in order for $A$ and $B$ to be independent.
I really don't understand what the question is asking. For two events to be independent, the probability of one of the events occurring must be independent of the other event occurring. So shouldn't the answer simply be $6$?