It is an Arithmetic Progression with a constant difference of 11 and first term 15.
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It is (5+37)/2 = 21.
Assuming the space is between the 5 and the 37, you could put 21 in there and all the numbers would be 16 apart from each other.
It is: (15+37)/2 = 26
26
Of that group, 19 and 37 are prime.