Why not make a simpler model and learn from it?
How many even integers are there between 1 and 11? Let n=11. Type out the even integers: 2, 4, 6, 8, 10. So, if n=11, the number of even integers between 1 and n=11 is (11-1).2, or (10)/2, or 5.
Make another model for, say, n=21. type out all the even integers between 1 and 21. Does our model work? It predicts (21-1)/2, or 10, even integers between 1 and 21.
Have fun!
