Discussion
How many twin primes are there in the numbers from 1 to 50 ?
Answer: A
There are 6 twin-prime pairs from 1 to 50.
Twin primes are pairs of primes that differ by 2. The primes up to 50 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47. The pairs differing by 2 are:
(3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43)
That is 6 pairs. 23 has no partner (21 and 25 are composite), nor do 37 and 47. Counting 5 or 4 pairs usually comes from missing (3, 5) or (5, 7), since 5 belongs to two pairs.
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