The mean, m, is computed as follows:
[tex]m=\frac{\text{ sum of the terms}}{\text{ }number\text{ of terms}}[/tex]Substituting with m = 9 and the data, we get:
[tex]\begin{gathered} 9=\frac{7+1+1+d+18}{5} \\ 9\cdot5=27+d \\ 45-27=d \\ 18=d \end{gathered}[/tex]