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RE: 97 percent will fail to answer this
Spoilers!!!
The answer is 45
1+4=5 => 1×(4+1)=5
2+5=12 => 2×(5+1)=12
3+6=21 => 3×(6+1)=21
5+8=x => 5×(8+1)=x
so x=45 :)
the rule followed above is 1st number multiplied by (2nd number + 1)