Can any expert person solve these probability questions???

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System A uses 4-digit personal secret numbers (i.e., 0000–9999), and on each login attempt it randomly chooses two of the numbers and asks the user to cite them.

(a) Suppose the attacker has not made any observations yet, and tries to log into Alice’s account by guessing the two digits requested. What is the probability that his guess of the two digits is correct?
(b) Suppose the attacker has made one observation of the credentials
Alice used to successfully log into her account, and now he tries
to log in by himself and is faced with a fresh request of two digits. Assume that he enters the requested digit if he knows it, and otherwise makes a random guess. What is the probability that he can enter them correctly?

(c) Suppose Bob has made two observations of the correct credentials.
What is the probability that he now knows all four digits of Alice’s
PIN?

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@vicky110,

a) Assuming they must be guessed in the proper order, 1/100.
b) Call the secret number ABCD. Without loss of generality, assume Bob saw AB. Bob could be asked for AB, AC, AD, BC, BD, or CD, each with probability 1/6. His chances for getting each correct are 1, 1/10, 1/10, 1/10, 1/10, 1/100, respectively. Therefore, his overall probability is 1.41/6 = 0.235.
c) Again, without loss of generality, assume the first two digits Bob saw were AB, he knows all four digits only if he then sees CD. The probability of that is 1/6.

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Can any expert person solve these probability questions???

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