An analyst is studying two corporate risks, $E_1$ and $E_2$. She estimates the probabilities of these risks occurring individually as $P(E_1) = 0.75$ and $P(E_2) = 0.85$. If no other information about their relationship is available, what is the tightest interval of possible values for the probability that both risks occur simultaneously, $P(E_1 \cap E_2)$?