Question 1207335
<pre>
PRICE         HOW MANY        COST
$2.5            n             2.5n
$8.5            1             8.5*1
                            ----------

                             2.5n+8.5 </pre>


Reported Average Price for All these pens combined
{{{(2.5n+8.5)/(n+1)=4.5}}}
and really want the final result for n+1.


{{{2.5n+8.5=4.5n+4.5}}}

{{{4=2n}}}

{{{n=2}}}

So the needed one more pen, 

He bought total of 3 pens.