Question 1140762: Bob received five email messages yesterday in the following order: A, B, C, D, and E. Whenever he opened his mail box, he always replied the most recent message first. Which one of the following is the possible order he replied to the five messages:
(1) ABECD
(2) BAECD
(3) CEDBA
(4) DCABE
(5) ECBAD
Answer by greenestamps(13200)   (Show Source):
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(1) Not possible. If he read E before either C or D, then at that time he had already received both C and D; he then had to read D before C.
(2) Ditto.
(3) Possible. He read C first, having received A, B, and C. While reading C, he received D and E; so next he read E and then D. Not having received any further messages, he next read B and then A.
(4) Not possible. If he read D first, then he had already received A, B, and C also. So he can't read A before B.
(5) If he read E first, then he had already received all 5 messages; the order he read them would have to be EDCBA.
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