Can You Complete This Numeric Serie? 95% Failed To Solve

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[joansoldevilacaba]

12. Alternative sum and subtraction. 7 and 5 respectively

[bosoxfan]

Up 7, down 5,
Up 7 & down 5 again,  
Up = 12

[MrMousley]

Before I watch your video I would say 12
Alternate numbers increase by 2
1 3 5 and 8 10 12

[jrjamieson]

The relationships between I and II, III and IV, and V and VI are the same.

1+x=8, x=7
3+x=10, x=7
5+x=12, x=7