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Your LimeSurvey version: 5.2.12
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We have multiple choice question for assessment and we have to calculate score depending on the choices selected like "none of 1,2 and 4 selected" value is 1.5 OR "Atleast one of 1, 2, and 4 are selected" value is 3 OR "All are selected" value is 4

Hi,
so it is a question of txpe "multiple".
Obviously there is no subquestion coded "3". Or is it?
Let's call your question Q1 and the subquestions are coded "1", "2", "3", "4","5","6",...
In an equation you use a nested IF-statement and count the answers as explaind in the manual, like
{if(count(Q1_1,Q1_2,Q1_4)==0,1.5,if(count(Q1_1,Q1_2,Q1_4)==1 or count(Q1_1,Q1_2,Q1_4)==2,3,if(count(Q1_1,Q1_2,Q1_4)=,4,0)))}

As you nearly never show everything needed ee do not know how your real question looks like
Because if you only have subquestion codes "1", "2","4" it is shortened to
{if(count(that.Q1)==0,1.5,if(count(that.Q1)=,4,3))}