Question

You have 50 rupees, and how it works? Spend Amount Balance amount 20,30 bal 15spent 15 bal, 9 spent 6 bal 6spent 0 bal Total= 50 Total=51 No?

Answer 1

50-20=30
30-15=15
15-9=6
6-6=0         
20+15+9+6=50

Answer 2

  50  -   20  =  30
  30  -  15  =  15 
  15  -   9   =   6
   6   -   6   =  0
============
101  -  50  =  51

The problem says we get 51 when we add the balances.  but we must get 50 - is the idea.

However,  we should only add the spent amounts.  We cannot add the balances.. See the following example I give here.   In this, start with 50 and spend 10 in 5 steps.  Then balances will be 40, 30 etc.

  50  -  10   =  40 
  40   -  10  =  30
  30  -  10   = 20
  20  -  10  = 10
  10  -  10  =  0
=============
         50    =  100