Hi
I have a below requirement,
I have created an object for calculating mileage vs cost for bike
I have the below fields in my object:
 Object Name:
API NameMileage_vs_Cost__c
Bill AmountBill_Amount__c
Current ReadingCurrent_Reading__c
DateDate__c
Past ReadingPast_Reading__c
PetrolUserName__c
Petrol User is a master object for( Mileage_Vs_ Cost__c) object
Example : For the first time when I save my record, current reading is zero fueled on 3/21/2016,
I will be travelling whole month and next time when I am ready to fuel the petrol , I will be entering some 1000 kms as current reading and when I do this process I want the past reading(i.e; previous month's current reading which is 0(zero) when I filled fuel on 3/21/2016) to  get populated in Past_reading__c filed of my record….
So that I can calculate mileage = (Current reading - past reading) and based on this further developments I can do on the page; can any one help me in this requirement I feel grateful and I am trying it using List and triggers and am failing…