The z score is given by:
[tex]\begin{gathered} z=\frac{x-\mu}{\sigma} \\ \text{ where:} \\ \mu\text{ is the mean} \\ \sigma\text{ is the standard deviation} \end{gathered}[/tex]In this case the mean is 63, the standard deviation is 10 and Roberto's score is 90; plugging these values we have that:
[tex]\begin{gathered} z=\frac{90-63}{10} \\ z=\frac{27}{10} \\ z=2.7 \end{gathered}[/tex]Therefore, Roberto's z-score is 2.7
