What percent of the first 20 natural numbers are one-digit numbers?

It depends on how you define natural numbers. There are two possible definitions.

1. All non-negative integers.
   one-digit natural numbers: 0 - 9 for a total of 10
   first 20 natural numbers: 0  - 19 for a total of 20
   10/20 = 50%

2. All positive integers.
   one-digit natural numbers: 1 - 9 for a total of 9
   first 20 natural numbers: 1 - 20 for a total of 20
   9/20 = 45%