Simple Trick

We are just indirectly dividing 7 by 2 as the original no is nothing just 3.5 more than original no which is half of 7

Oh i love these games.....used to play them back in the day !!!!!

let the number be x, then what we do is -

\frac { x\times 2+7 }{ 2 } -x\quad =\quad x+\frac { 7 }{ 2 } -x\quad =\quad \boxed { 3.5 }