A school librarian counts the books borrowed on eight school days: 9, 4, 6, 12, 14, 7, 11 and 5…

Step 1. The mean. $\bar{x} = \dfrac{68}{8} = 8.5$ books.

Step 2. The range. The largest value is $14$ and the smallest is $4$, so the range is $14 - 4 = 10$ books.

Step 3. The distances. The distances from $8.5$ are $0.5, 4.5, 2.5, 3.5, 5.5, 1.5, 2.5, 3.5$. They add to $24$.

Step 4. The mean deviation. $\text{MD} = \dfrac{24}{8} = 3$ books.