How to restrict number of times a column in array can be selected?

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I have an Array type question (QArray), which has 5 columns with the answer codes 1 through 5.
I want to restrict the total number of responses to the column with the answer code "5" to 3 -- that is, no more than 3 options can be rated "5". The sub-questions are coded 1 through 15.
Another way to explain it, column 5 in my array has the heading "High Priority" and then the guidance I give the respondent is: A maximum of 5 responses may be selected as “High Priority” -- and I want to enforce this so that no more than 5 rows can be marked as "high priority"...

Is there a way to do this? 

I should say:
validate like

countif("5",self)<4

 

Brilliant -- does the job nicely! From that, I also added a validation text :

{if(countif("5",self)<4,'(A maximum of 3 responses may be selected as High Priority)','You can only select a maximum of 3 High Priority Responses')}

So the respondent knows what's up

I didn't mention the  validation tip, as you are an experienced user who knows this.
So I only provided the equation.

Regards

Joffm