According to the properties of equality,
[tex]\begin{gathered} a=b \\ \Rightarrow a\cdot c=b\cdot c \end{gathered}[/tex]
In our case,
[tex]\begin{gathered} \frac{x}{3}=5 \\ \Rightarrow3\cdot\frac{x}{3}=3\cdot5 \\ \Rightarrow3\cdot\frac{x}{3}=5\cdot3 \end{gathered}[/tex]
Therefore, the missing number is 3.
