摘要：
A reversible prime in any number system is a prime whose "reverse" in that number system is also a prime. For example in the decimal system 73 is a re 阅读全文
摘要：
A long-distance telephone company charges its customers by the following rules: Making a long-distance call costs a certain amount per minute, dependi 阅读全文
摘要：
Suppose a bank has K windows open for service. There is a yellow line in front of the windows which devides the waiting area into two parts. All the c 阅读全文