Ok then I tried all that google and wiki stuff and it is way over my head and APR does not apply to this because an apr assumes an annual rate of interest paid in increments. So I will respond to your *facetiousness* by re-asking the question. Take note that my degree is in English not maths. So you can all understand my question, here is MY War and Peace

I have a small investment growing at 50% per month. This is not an apr it is per month. I was told that my investment would grow 100 fold in 12 months. You already helped me in confirming that by making sure I was using the exponential growth formula a(1+r)^{12} correctly and it confirms that my investment will grow at least 100 fold in the 12 months.

Now, after year 1 let's say my investment is worth €2000 (it started with €20). That will grow at the same 50% monthly; but I may take an income of say €500 a month out. This will obviously affect the growth. All I wanted to know was is there a simple adjustment to the formula to allow for these deductions. If it is too complicated, don't worry about it. I can work it out in columns and rows on paper or in a spreadsheet. I just hoped there might be an easier way of working it out. If there isn't - all you have to do is say so and I will go away.

Les

You will need to talk to a financial advisor (or an officer of the financial institution} to get precise answer.

The answer will depend on;

How do your financial institution calculate interest rate - APR, APY, daily, or some other method?

When do your financial institution calculate interest rate and add to the principal?

When (date) do you withdraw your money relative to the date above?

The answer is not complicated - but requires critical reading of contract (that degree in English might come-in handy here).