2.若每天只進8個定便當,則賣不完的機率為多少?

A random variable X represents the number of defective mobile phone is a shipment of four iPhones to a local mobile phone stor(E) Assume that each mobile phone is equally likely to be defective or non-defective, and also assume that each mobile phone is defective or non-defective independently of the other mobile phones. Then the expected number of defective mobile phones is _______.