Let $t$ be TNYWR.

The average of the list of five numbers $13,16,10,15,11$ is $m$.

The average of the list of four numbers $16, t, 3,13$ is $n$.

What is the value of $m-n$ ?
The average of the first list is $m=\frac{13+16+10+15+11}{5}=\frac{65}{5}=13$.

The average of the second list is $n=\frac{16+t+3+13}{4}=\frac{32+t}{4}=8+\frac{1}{4} t$.

Therefore, $m-n=13-\left(8+\frac{1}{4} t\right)=5-\frac{1}{4} t$.

Since the answer to (a) is 4 , then $t=4$, and so $m-n=5-1=4$.