Respuesta :

31 - as the difference (starting from 1) increases by 1

so:

3,+1=4,+2=6,+3=9,+4=13,+5=18,+6=24,+7=31,
31.

3+1=4
4+2=6
6+3=9
9+4=13
13+5=18
18+6=24
24+7=31
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