The five triangular numbers that follow 21 are 28, 36, 45, 55, and 66.
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3 is a triangular number that is also prime.Here is a list of some of the first few triangular numbers:0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595, 630, 666, 703, 741, 780, 820, 861, 903, 946, 990, 1035, 1081, 1128, 1176, 1225, 1275
1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435,
The prime numbers from 21 to 40 are 23, 29, 31, and 37.
Out of those 100 numbers, 21 of them are prime numbers, so the remaining (100 - 21) = 79 are composite.
There are 3 prime numbers from 21 to 35:23, 29 and 31