Someone borrows £10 off me every day for 1 year. I charge simple interest (8% per annum in other words 8%/365 per day) on that £10. At the end of the year, I want to know how much interest the debtor needs to pay me in addition to the principal (which is 365 * £10 = £3650).

So far I have got to this in my thinking:

P = principal

i = daily interest which is P * 0.08/365

d = 365 (being the difference between the start and end date)

T = total interest for period

T = (P*i*d) + (2P*i *(d-1)) + (3P*i*(d-2)) + ((4P*i*(d-3)) etc

I am using Excel for this. My actual data spans a much longer period than 1 year, so I can't type this out. Plus I have about 3,000 amounts I need to do this calculation on.

Please Please help!