Question

Unathi thinks of a Natural number, doubles it, adds 1 and squares the result. If he ends up with 81,what is the number he thought of?

Answer 1

$(x\cdot2+1)^2=81$ ;

$x\cdot2+1=\sqrt{81}=|\pm9|=9$ (not (-9), because x is a natural number);

$x\cdot2=9-1=8$ ;

$x=\frac{8}{2}=4$.

Answer: 4 is the number he thought of.