Prime Numbers 素 数 之 迷
How many two digit prime numbers are there in which both digits are prime numbers? (For example, 23 is one of these numbers but 31 is not, since 1 is not a prime number.)
-------------------------------------------
上期答案: Answer: (a)
Solution1: Experimenting with the numbers in turn, dividing 6 into15 and 10 into15, we get answers one whole number apart, so they are together again at 15 minutes.