Question 1190755
.



            Solve backward  MENTALLY.



<pre>
    30 minus 12 is one-half of the square,

or

    18 is one-half of the square


Hence, the square is two times 18, i.e. 36.


It means that the number is the square root of 36, i.e. plus or minus 6.


<U>ANSWER</U>.  The number is 6 or -6.
</pre>

Solved mentally.