If the ratio of length to width is 16 : 9, we take the same order to be each one, so length : width, that is 16 length and 9 width.
So, if we have a length of 90, if w is its width, then the ratio 90 : w must be the same as 16 : 9, so:
[tex]\begin{gathered} \frac{90}{w}=\frac{16}{9} \\ 90\cdot9=16w \\ 16w=90\cdot9 \\ w=\frac{90\cdot9}{16}=\frac{810}{16}=50.625\approx51 \end{gathered}[/tex]
So, the width must be approximately 51 feet, alternative A.
