Answer:
2
Explanation:
The mean of a set of numbers is also known as the average.
Finding the average is adding all the numbers and then dividing by however many numbers you added together.
We know the mean is 4
We know two numbers are 1 and 9
Since we know that we have 3 total numbers, we can work backwards... starting by multiplying 4x3
4x3 = 12
Now to find that 3rd number, we can take 12 and subtract 9 and 1 (your other two numbers)
12-9-1 = 2
Making that missing number 2