Subtract 4 each time.
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The 13 should be 31... then 1+1+1=3 1+1+3=5 1+3+5=9...
18. The pattern is add 5, then subtract 1.
1 5 9 13 17 21
+5
There are 10 possible combinations so 60 possible permutations. The combinations are:(1, 3, 21), (1, 5, 19), (1, 7, 17), (1, 9, 15), (1, 11, 13),(3, 5, 17), (3, 7, 15), (3, 9, 13), (5, 7, 13), (5, 9, 11).