SOLUTION
From the question, we can see that this is a ratio. The biologist wants to make a feed that contains cod and mackerel in the ratio of 8 : 7, that is 8 parts of cod and 7 parts of mackerel.
An acceptable mixture of this is also
[tex]\begin{gathered} 8\times3\text{ parts of cod }=\text{ 24 parts of cod } \\ 7\times3\text{ parts of mackerel }=\text{ 21 parts of mackerel} \end{gathered}[/tex]Hence an acceptable mixture of this is 24 parts of cod and 21 parts of mackerel.