Question

a person has total money was 50.he spent 20 rupee on lunch that he has now 30 rupee.then he eat a chip that cost was 15 then he was 15 rupee now.then he eat a ice cream which cost 9 rupee then he now only 6 rupee.then he eat toffee which cost 6 then he had no rupee ( 0 rupee) . friends draw a diagram in center total money left side his balance right side he spend money on..then u total both side valance and spend money. balanced was 51 and spend was 50 why friends ???

Answer 1

do the sum of amount spent not the balance amount so 
      Spent      Balance
          0              50 
         20             30
         15             15
          9               6
          6               0
total   50             51

Answer 2

S = Rs 50   

    Lunch = Rs 20.    =>  Balance = Rs 30.
    Chips = Rs 15.    =>  Balance = Rs 15.
   Ice cream = Rs 9  =>  Balance = Rs 6.
   Toffee = Rs 6        =>  Balance = Rs 0


       Spent       Total      Balance
           20          50          30  = 50 - 20  
           15          30          15  = 50-20-15
            9          15            6  =  50-20-15-9
           6            6            0  =  50-20-15-9-6
  ==================================
Total  50        101          51   =  4 * 50 - 4 * 20 - 3 * 15 - 2 * 9 - 6

What matters is that  Sum of totals = sum of Spent and sum of Balances.  It is correctly balanced.

Sum of spent will always be equal to the initial total, when the Final balance is 0.
This is what matters.  Totaling balances and the totals is of no consequence.  It need not follow any rule.