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Computer Science 70 - Fall 2001 - Wagner - Midterm 2

# Computer Science 70 - Fall 2001 - Wagner - Midterm 2 - cs70...

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cs70, midterm 2, fall 2001 file:///C:/Documents%20and%20Settings/Jason%20Raftery/Desktop/R... 1 of 3 5/12/2006 8:42 PM cs70, fall 2001 midterm 2 professor wagner Problem #1 (18 pts.) Short-answer questions In each of the following, no justification is required. However, if you give a brief explanation of your answer, we may assign partial credit based on your explanation (if your answer is incorrect), and we reserve the right to deduce points for an incorrect explanation (even if your answer is correct). (a) [4 pts.] Alice flips a fair coin n times in a row. What is the probability that she receives exactly i heads? (b) [4 pts.] E and F are events. Also, Pr[E] = 0.2 and Pr[F] = 0.3. What is the smallest value that Pr[E union F] could possibly be? What is the largest it could be? (c) [5 pts.] How many 2001-bit strings are there that do not contain 01 anywhere as a substring? (d) [5 pts.] Let n range over the positive integers such that n = 11 (mod 12), and define z = (n+1)/2. For which n is z invertible modulo n? Problem #2 (12 pts.) Digit sums

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