Answer: 1
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Step-by-step explanation:
To average a bunch of numbers, we add them up and divide by how many numbers there are. In this case, we'll divide by 3 since there are 3 numbers.
Add up the values: m+n+(-1) = m+n-1
Divide by 3: (m+n-1)/3
Set this equal to 0 since we're told the average is 0: (m+n-1)/3 = 0
Now let's isolate m+n. It might help to replace "m+n" with another variable, say P to get (P-1)/3 = 0.
Solve for P
(P-1)/3 = 0
P-1 = 3*0 .... multiply both sides by 3
P-1 = 0
P = 1 ... add 1 to both sides
m+n = 1 .... replace P with m+n
So that's why the answer is 1