If you are talking about any number then.....
imagine a mountain, any number less than 5 is only the left and any number greater than 5 is on the right
Here, for example, let's try 13
look at 13 on the pyramid .... since it's not a the top of the mountain so it falls back down to 10... it rounds down to 10
Now let's try 18 ...hmmm.... 18 is not at the top either so it falls as well. It falls down to 20 it rounds up to 20
how about 15 u might be asking...it is at the top...15 is a little different ...make sure no one is looking and push it off the right side and falls to 20. Also rounding up to 20
5
4 6
3 7
2 8
1 9
0 10