public class avgCase{

    private final Case cas;

    public double avrgTime{public get; public set;}
    public double count{public get; public set;}
    public double d{public get; public set;}
    
    public avgCase(ApexPages.StandardController controller) {

    this.cas = (Case)Controller.getRecord();
    List <case> allCase=[Select Case_Duration__c from case where accountid=:cas.accountid and status='Resolved'];

    double d=0.0;
    integer count=0;
    if(allCase.size()>0){
    for(case c: allCase)
    {
     d+=c.Case_Duration__c;
     count++;
    }
    }
    if(count>0){
        avrgTime=d/count;
    }
    else
    {
        avrgTime=0;  //This is the line which is not getting code coverage
    }
    }
}

@isTest
public class avgCase_test{
    static testMethod void testAvgCase() {
        Account a = new Account(name='TestAcc',Reseller_Id__c=15243, Virtualisation_Id__c=1);
        insert a;
        Case c = new Case(subject='Test Case',AccountID=a.id,status='Resolved');
        insert c;
        ApexPages.StandardController sc = new ApexPages.standardController(c);
        avgCase testMe = new avgCase(sc);
        testMe.d=1;
        testMe.count=1;
        testMe.avrgTime=1;
        ApexPages.currentPage();
        System.AssertEquals(1, testme.count);
        System.AssertEquals(testme.avrgTime, 1);
	}   
}