Answer: M = 600.
Explanation: We know the average of these numbers is 400. To find the average, we add all the given numbers and divide it by how many numbers there are. This would look like:
100 + M + 500 = ?
? / 3 = 400
400 • 3 = 1,200
We know 1,200 = “?”
Now, let’s plug it in:
100 + M + 500 = 1,200
600 + M = 1,200
1,200 - 600 = 600
M = 600
We can plug 600 in for M to see if it verifies:
100 + 600 + 500 = 1,200
1,200 / 3 = 400
Therefore, our answer is correct.
Final answer: M = 600