21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39.
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All those not less than 20.
and the answer is 2,3,11,13,and 17.
All the prime numbers less than 20 are 2,3,5,7,11,13,17,19
A more formal definition for a number less than 20 could be, "a number x, x < 20" or if you want to express the set of all numbers less than 20 "{x | x < 20}" which is read "the set of all numbers x such that x is less than 20". In interval notation, you can write (-infinity, 20) where "infinity" should be replaced by the infinity symbol.
23 and 29 are the two prime numbers greater than 20 but less than 30.