Suppose you're sitting at a cafe with your friend Dharmendra, and you realize that you haven't heard his iPhone ring at all in the 10 minutes since he showed up. This is a bit surprising, since in your experience Dharmendra's phone is almost always ringing. You begin to wonder whether his phone might be off. You decide to put some numbers to this. Let's suppose that when Dharmendra's phone is on, there's an 75% probability that it will ring in any given 10-minute interval. Also, at any given point in time, in the absence of any other information, there's a 90% probability that his phone is on. (And in the current situation, it's either been on the whole time or off the whole time, since you haven't seen him take it out.) You do have additional information, since you know the phone hasn't rung. Given that you haven't heard it ring in the past 10 minutes, what's the probability that Dharmendra's phone is off? Give an explanation for your answer using Bayes' Rule.