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Complete the series; 3, 4, 7, 8, 11, 12, ______?

Complete the series; 3, 4, 7, 8, 11, 12, ______?

Explanation

3+1 = 4

4+3 = 7

7+1 = 8

8+3 = 11

11+1 = 12

12+3 = 15