Arithmetic Average exchange rate

Does it exist or is it possible to create an exchange rate type which is based in an arithmetic average of the exchange rate between two currencies?, for example:

Local currency is euro: I enter a transaction with the exchange rate 1 usd=1,5euro

and after i enter a transaction with the exchange rate 1 usd=2 euros, so the exchange rate for the next transaction would be 3,5/2=1,75.

Thanks in advance

Marc