Quant help please

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i keep getting the answer as 11.
3 + 4 = 7
3 + 8 = 11
3 + 12 = 15
3 + 16 = 19
6 + 4 = 10
6 + 8 = 14
6 + 12 =18
9 + 4 = 13
9 + 8 =17
12 + 4 = 16
15 + 4 = 19

where am i going wrong?

19 will be count only once{(3,16),(15,4)} gives us the same integer 19

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