This mental arithmetic should be done in your head, in the order in which you read the question and without using calculator, pen, paper etc. I put it in last night but thought it could get another run out here.
Take 1000 and add 40 to it. Now add another 1000. Now add 30. Add another 1000. Now add 20. Now add another 1000. Now add 10. What is the total?