Expense and Balance Mismatch

Expense and Balance Mismatch

I searched and couldn't find a post in stack exchange for this. I recently
saw a calculation for rupee spend and having balance in yahoo, but i am
not quiet convinced with the answer given...
the question goes like this....
If i have 50 Rupees
Spend -- Balance
0---------50
20--------30
15--------15
9---------6
6---------0
If we see the sum of amount spend it gives 50 Rupees.
But if we calculate the balance amount (30+15+6) it gives 51 rupee instead
of 50 rupees.
How could this be possible....
If this question is mentioned somewhere else in this forum please refer
that too...