Let's denote the first three positive numbers as $1, 2, 3$.

The sum of these numbers is: $1 + 2 + 3 = 6$

Now, it's given that one of these numbers was missed, and the average of the remaining two numbers is 9. Let the missing number be $x$.

The sum of the remaining two numbers (after missing one) is: $1 + 2 + 3 - x = 6 - x$

The average of the two remaining numbers is given as 9, so we can set up the equation for the average: $\frac{6 - x}{2} = 9$

Multiplying both sides of the equation by 2: $6 - x = 18$

Solving for $x$: $x = 6 - 18 = -12$

Therefore, the missing number is $-12$.

Would you like further clarification on any part of the solution, or need help with additional questions?

Here are 5 related questions for practice:

1. What is the average of the first 5 positive numbers?
2. If the average of 3 numbers is 10, what is their total sum?
3. How would the average change if a number larger than 3 is added to the first three positive numbers?
4. What is the sum of the first 100 positive integers?
5. How do you find the median of a set of numbers?

Tip: Always check the problem carefully for clues on how many numbers are being averaged or missed, as it affects how you set up the equation.