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%