30, 42, 66, 70 and 78.
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There are 4 prime numbers under ten.
They are: 3+5+11 = 19
There are 15 prime numbers less than 50.
Such a number would either have to be a perfect cube of a prime number, such as 33 = 27 (the only one in the specified range), or the product of exactly two different prime factors, such as 2 x 3, 2 x 5, 3 x 31, etc.
By definition, a product of two primes has atwo factors (other than 1 and itself) and so it cannot be prime. So the set is not closed.