@waitmaya I think the answer is 31.
1 plus 2 equals 3, 3 plus 4 equals 7, 7 plus 8 equals 15. Based on the pattern of the numbers added each time, it should be 2 to the power of n(n refers to where it gets added). So 15 should be plus two to the power of four, which is 15+16=31.