<p>ai let me just give u a hint, if u choose a number between 99-90 </p>
<p> </p>
<p>the number can be discribed as (90+X); so if u add the two digitals, it will be (9+X)</p>
<p> </p>
<p>and now substract from the original: (90+x)-(9+x)= 81 </p>
<p> </p>
<p>so this is math. </p>
<p> </p>
<p>end of story.</p>
<p> </p>
<p>thank you</p>