Rounded to hundred how can i round to hundred

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