The density is a constant ratio between the number of fish and the volume of the tank, that is
[tex]\text{density}=\frac{\#of\text{ fish}}{volumeft^3}=0.5[/tex]since this ratio must be preserved, then we can write
[tex]\frac{30\text{ fish}}{\text{V}}=0.5[/tex]where V denotes the unknown volume. Now, we need to isolate V.
By multiplying both sides by V, we have
[tex]30\text{fish}=V\times0.5[/tex]and by dividing both sides by 0.5, we get
[tex]\frac{30\text{ fish}}{\text{0.5}}=V[/tex]which gives
[tex]\begin{gathered} V=\frac{30}{0.5}ft^3 \\ V=60ft^3 \end{gathered}[/tex]Therefore, the answer is option A: 60 cubic feet
