In the formula of the volume given, we have the value of the volume, the length and the height, so we need to calculate the value of the width:
[tex]\begin{gathered} V=l\cdot w\cdot h\to w=\frac{V}{l\cdot h} \\ V=90,l=10,h=3 \\ w=\frac{90}{10\cdot3}=\frac{90}{30}=3 \end{gathered}[/tex]So the width is 3.