Counting users while using Fractile

[#1]

Hello Experts,

May I request your help  for the following?

I have the following fields

PRODUCT_CODE, USER_ID, BALANCE

I have created a chart with following Parameters

Dimension =  PRODUCT_CODE

Expression1 = Fractile(BALANCE, 0.1)

Expression2 = Fractile(BALANCE, 0.2)

Expression3 = Fractile(BALANCE, 0.3)

Expression4 = Fractile(BALANCE, 0.4)

Expression5 = Fractile(BALANCE, 0.5)

Expression6 = Fractile(BALANCE, 0.6)

Expression7 = Fractile(BALANCE, 0.7)

Expression8 = Fractile(BALANCE, 0.8)

Expression9 = Fractile(BALANCE, 0.9)

Expression10 = Fractile(BALANCE, 1)

I got the chart as follows

Now I want to Count. the user ID's for associated Balance.

If I create a new chart with the following is not working.

DImenion= PRODUCT_CODE

Expression1= fractile( TOTAL aggr(count( BALANCE),USER_ID), 0.1)

Expression2= fractile( TOTAL aggr(count( BALANCE),USER_ID), 0.2)

Expression3= fractile( TOTAL aggr(count( BALANCE),USER_ID), 0.3)

How do I count?

Thank you

[#2]

Please help me.

I have exam data of a class of 50 students.

Now I want to calculate the average (and, median) of top 80% students of that class.

How can I do that?
Please help.

Thanks.