Simple
like when you buy something at the store. the person is selling products to you
After every month's withdrawals, 4/5 of the original amount at the start of the month will remain. The amount at the start of every month will change. Thus:
an = 4/5 (an-1) ; where a1 = 500.
1,6,11,16,21,26 is what you would get if you add 5.
The tools couldn't have made the proper measurments
