Problem with creating a proper formula with missing values


I have a problem with creating a proper formula for creating a sum score of 11 questions. (using SPSS 20.0)
I use questions wich people can score a 0, 1, 2 and 3. This scale uses 11 questions.

The formula i have to use to create this score is:
(total score of 11 questions) / (3* the answered questions) * (100)

The first part i understand, i have to use Sum(W1, W2,....W11)
But then i have for example 1 missing value and my formula uses (3*11) and that's wrong, because i have to use (3*10).

So my question is: how can i insert the function of ''aswered questions'' in my formula instead of the exact nummer of 11 which i'm using now.

I hope someone could help me with this!

Many thanks from a Dutch graduating student!