Hello All,
I have completed this challenge.

1-For this first you need to create a helper formula field(type-percent) 
Percent Completed :
(DATEVALUE( CreatedDate ) - CloseDate )/100

2- Then you need to create the actual formula field (type- text) by using the helper formula field.
Opportunity Progress :

IF( Percent_Completed__c <=25,"Early", 
IF(Percent_Completed__c <=75,"Middle", 
"Late"))

Thanks,
Nida