Adding List elements to a variable

Hi All,
I have a List<ART_Case__c> where ART_Case is a custom Object which has a field First_Response_Time__c having data type Number which Stores Average Response Time for Cases.Now I am Encountrig problem in a piece of code.

a=[Select First_Response_Time__c from ART_Case__c where Queue_Name__c=:ownerMap.get(c.OwnerId) and Created_Date__c=: date.Today() ];
       
       Float j=0.00;
       for(Integer i=0;i<z;i++)
       {
           j=j+a[i];
       }

The problem is with the data typeof variable j.Error: Compile Error: Invalid type: Float at line 77 column 8.Please Suggest me which Data Type I Should Use
Thanks in Advance