What is the probability that we choose a number that is divisible by 3 among the first 20 natural numbers? Justify your answer

To find the probability of choosing a number that is divisible by 3 among the first 20 natural numbers, we first need to find how many numbers among the first 20 are divisible by 3.

The first multiple of 3 is 3, and the last multiple of 3 that is less than or equal to 20 is 18.

So, the numbers divisible by 3 among the first 20 natural numbers are 3, 6, 9, 12, 15, and 18. There are a total of 6 numbers that are divisible by 3.

Now, the total number of choices is 20 (from 1 to 20).

Therefore, the probability of choosing a number that is divisible by 3 among the first 20 natural numbers is given by:
\text{Probability} = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} = \frac{6}{20} = \frac{3}{10} = 0.3

\boxed{0.3} is the probability of choosing a number that is divisible by 3 among the first 20 natural numbers.