How many points are there on the globe where, by walking one mile south, then one mile east and then one mile north, you would reach the place where you started?
Take number 1000 and then add 20 to it.
Now add 1000 one more time.
Now add 30.
Now add 1000 one more time.
Now add 40.
Now add 1000 one more time.
Now add 10.