24. Albert and Bernard just become friends with Cheryl, and they want to khow when her birthday is. Cheryl gives them a list of 10 possible dates. May 15 J
Lời giải 1 :
Of the 10 days that Cheryl gave, from the 14th to the 19th of every month, the 18th and 19th appear once. If Cheryl's birthday is on these two days, Bernard already knows the answer ( eliminate May 19 and June 18)
If Cheryl tells Albert her birth month is May or June, Cheryl's birthday could be May 19 or June 18 => Bernard will know the answer.
But Albert says that Bernard doesn't know, which means Cheryl tells Albert that her birth month is July or August (eliminate May 15, May 16 and June 17)
At first, Bernard did not know Cheryl's birthday but he knew it after Albert's first statement because:
Among the remaining days, from July 15-17 of July or August, the 14th appears twice.
If Cheryl tells Bernard his birthday is on the 14th, he won't know the answer. But Bernard knows, so we eliminate July 14 and August 14. (Eliminate July 16, August 15, and August 17 left)
After Bernard's sentence, Albert also knew the answer. If Cheryl told Albert her birthday was in August, Albert doesn't know because there are two days in August.
So Cheryl's birthday is July 16.
Thảo luận
Lời giải 2 :
Câu trả lời: Of the 10 days that Cheryl gave, from the 14th to the 19th of every month, the 18th and 19th appear once. If Cheryl's birthday is on these two days, Bernard already knows the answer ( eliminate May 19 and June 18)
If Cheryl tells Albert her birth month is May or June, Cheryl's birthday could be May 19 or June 18 => Bernard will know the answer.
But Albert says that Bernard doesn't know, which means Cheryl tells Albert that her birth month is July or August (eliminate May 15, May 16 and June 17)
At first, Bernard did not know Cheryl's birthday but he knew it after Albert's first statement because:
Among the remaining days, from July 15-17 of July or August, the 14th appears twice.
If Cheryl tells Bernard his birthday is on the 14th, he won't know the answer. But Bernard knows, so we eliminate July 14 and August 14. (Eliminate July 16, August 15, and August 17 left)
After Bernard's sentence, Albert also knew the answer. If Cheryl told Albert her birthday was in August, Albert doesn't know because there are two days in August.
So Cheryl's birthday is July 16.
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