Answer
-3
Work Step by Step
In a polyatomic ion, the sum of the oxidation numbers of all atoms is equal to the overall charge on the ion.
The overall charge on $NH_{4}^{+}$ is $+1$.
Let the oxidation number of nitrogen be $x$.
The oxidation number of each hydrogen atom is $1$.
Therefore, we can write
$x+4(1)=1$
Or $x=1-4=-3$
