Discussion
How many pairs of twin primes are there between the integers 1 to 100 ?
Answer: D
There are 8 pairs of twin primes between 1 and 100.
Twin primes are pairs of primes that differ by 2. Listing them:
(3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43), (59, 61), (71, 73).
That is 8 pairs. Other near misses are not twins because one member is composite: 23 and 25 (25 = 5 × 5), 47 and 49 (49 = 7 × 7), 89 and 91 (91 = 7 × 13). Forgetting that 5 belongs to two pairs, (3, 5) and (5, 7), or treating 91 as prime, leads to the wrong counts 7 or 9.
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