averaging varying column entries

Hi
been away from this type of work for a while so am finding myself very rusty


have weekly data pulled from transport figures to give mpg figures, which are then averaged to give a figure for the 4week or a 5week month.

Sheet then displays highest and lowest results in range by driver name


I have just noticed an anomoly when the vehicle is NOT used and returns a no value in the cell range. In this instance the average formula should use the number of weeks there is a figure for as the base for the average

entry line would look like

wk1 wk2 wk3 wk4 AVE
5.1 5.8 6.0 5.6 AVERAGE(WK1:WK4) 5.625
5.1 x 6.0 5.6 AVERAGE(WK1:WK4 - divided by 3 in this case) 5.56 not 4.175

any help appreciated