(a) The ratio of starfish to sea horses is given as 7 starfish for every 5 sea horses. Therefore, the ratio can be expressed as:
\[ \text{Ratio of starfish to sea horses} = 7:5 \]
(b) If there are 15 sea horses, we can use the ratio to find the number of starfish. The ratio of starfish to sea horses is 7:5, meaning for every 5 sea horses, there are 7 starfish.
Let \( x \) be the number of starfish. We can set up a proportion based on the ratio:
\[ \frac{7}{5} = \frac{x}{15} \]
Now we can cross-multiply to solve for \( x \):
\[ 7 \cdot 15 = 5 \cdot x \ 105 = 5x \ x = \frac{105}{5} \ x = 21 \]
Thus, if there are 15 sea horses, there are 21 starfish in the aquarium.