Let $t$ be TNYWR.

The average of the five numbers $12,15,9,14,10$ is $m$.

The average of the four numbers $24, t, 8,12$ is $n$.

What is the value of $n-m$ ?
The average of the numbers in the first list is $m=\frac{12+15+9+14+10}{5}=\frac{60}{5}=12$.

The average of the numbers in the second list is $n=\frac{24+t+8+12}{4}=\frac{44+t}{4}=11+\frac{1}{4} t$.

Therefore, $n-m=\left(11+\frac{1}{4} t\right)-12=\frac{1}{4} t-1$.

Since the answer to (a) is 8 , then $t=8$, and so $n-m=2-1=1$.