You Have an Important Test Tomorrow Morning and Want to Be

Multiple Choice

You have an important test tomorrow morning and want to be sure to wake up at 6:00 AM to study.Without an alarm to wake you up, you know you have no chance.You have an alarm clock that works 80% of the time.To be on the safe side, you decide to set your alarm clock plus the alarm clocks that you borrow from some of your friends in the dorm.Assuming that each of these borrowed clocks is also 80% reliable, how many clocks should you borrow if you want to ensure that there is no more than 1 chance in a thousand that you will not be awakened at the proper time? (Assume statistical independence.)

A) 6
B) 4
C) 3
D) 5
E) 2