public class BatchClosedResponse implements Database.Batchable<sObject> {

   public Database.QueryLocator start(Database.BatchableContext BC){
         string query = 'Select id, First_Response__c,'+ 
                            'ClosedDate ,'+ 
                            'OwnerId, CreatedDate, Status,'+ 
                            'Helper_Closed_Date__c, '+
                            'LastModifiedDate, '+
                            'Helper_First_Responce__c '+
                            'from case'; 
                            
       System.debug('query'+query);
      return Database.getQueryLocator(query);
   }
    public void execute(Database.BatchableContext BC, List<Case> scope){
        List<Case> caseList = new List<Case>();
         BusinessHours defaultBH = [ SELECT Id 
                                   FROM BusinessHours 
                                   WHERE IsDefault = true  
                                   Limit 1
                                  ];
       for(Case caseObj : scope ){ 
         if( (caseObj.ClosedDate != Null)
                                          && caseObj.Status == 'Closed' 
                                         ){ 
                                             System.debug('I am here');
                                             Decimal lastResult = BusinessHours.diff( defaultBH.Id, caseObj.CreatedDate, caseObj.ClosedDate );
                                             
                                             Decimal lastResultingHours = lastResult/(60*60*1000);
                                             
                                             caseObj.Helper_Closed_Date__c= lastResultingHours;
                                             
                                         }
           caseList.add(caseObj);
                            }
        
        update caseList;
    }
    
    public void finish(Database.BatchableContext BC){

   }
}

@isTest
private class BatchClosedResponseTest {
    @testSetup 
    static void setup() {
         List<Case> lstCase = new List<Case>();
        
        for (Integer i=0;i<10;i++) {
           Case caseObj = new Case(Subject='Test', Status = 'New',ClosedDate =System.today()+10);
    lstCase.add(caseObj);
        }
        insert lstCase;
       
    }
    static testmethod void test() {        
        Test.startTest();
        BatchClosedResponse uca = new BatchClosedResponse();
        Id batchId = Database.executeBatch(uca);
        Test.stopTest();
         
    }
    
}