HI

Step 1 :

Find out the Maximum & Minimum values and it's percentages

Maximum 10% could be : Maximum - (Maximum *.10)
Minimum 10% could be  : Minimum + (Minimum*.10)

Step 2 :

Find out the count of Min and Max values like

Numbervar min;
if {No.of.Days} < Minimum + (Minimum*.10) Then
   min:=min+1;

Numbervar max;
If if {No.of.Days} <  Maximum - (Maximum *.10) Then
max:=max+1;

Step 3 :

Find out whether your min & max count is more than 1 or not and suppress those records through selection expert :

If min > 1 Then
{No.of.Days} < Minimum + (Minimum*.10)
Else
Minimum({No.of.Days}

Follow the same logic for Maximum.

You need to write a running total to arrive average.