How to calculator return?

How do you figure a return by percent in an account with the balance changing every month?

For example:

5% return for the month on a starting balance of $100,000
1% return for the next month on a starting balance of $90,000

Total return for the 2 months is $5900. What is the percent return for the 2 months?

Thanks
ThinAir

6.2%

You get that by first adding together all the starting balances, then dividing by the number of months. Then you simply do:

Gains / Starting Balance(the end result of the previous calculation) x 100

How do you come up with that figure?

Your starting balance is $100K, your profit is $5,900, your ROi is 5.9% (5900 / 100000 = .059)