Just counting numbers using fingers.
First all my ten fingers are closed. I will say 1 and open my one finger. Likewise for 6, all my right hand fingers are open and one of the left hand finger is open. So for 10, all 10 fingers are open. Now for 11, I will close all my right hand fingers(because all are open for 10) and open 1 finger in right hand. So for 11, 6 fingers are open. Likewise for 16, 6 are open. For 20, 10 are open. Likewise for 25, 10 are open.
Now number is the input for you and the number of open fingers is the output expected from you