3 is what my answer was
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Explanation:
From 5 to 5, we add 0
From 5 to 6, we add 1
From 6 to 8, we add 2
From 8 to 11, we add 3
Assuming this pattern keeps up, the next term is found by adding 4.
So 11+4 = 15 is the next term.
