Calculated Measure with conditions

jaymanivannan response is great.  To add to this, rather than using the / symbol and have to make a divide by zero check, just use the Divide() function.  I've also set this out using a spacing format, which is a good habit to get yourself into.

Here is an update

SalesAverage = DIVIDE(
        CALCULATE(
                SUM(Total Sales $), 
                [AccountCode]=410000
        ),
        CALCULATE (
                Sum(Total Sales Quantity), 
                [AccountCode]=420000
        )
)