Solution: Let the number of mackerel be $ m $. The ratio $ 5:4 = 20:m $ implies $ \frac{5}{4} = \frac{20}{m} $. Cross-multiplying: $ 5m = 80 $ → $ m = 16 $. There are $ \boxed{16} $ mackerel.

["Solution: Calculating the Number of Mackerel Using a Simple Ratio", "Understanding how to solve ratio problems efficiently is essential in math, and today we explore a clear, step-by-step solution to a common ratio question involving mackerel. By let m represent the number of mackerel, we apply a straightforward cross-multiplication method to determine the correct value.", "---", "### The Problem", "We are told:\nLet the number of mackerel be $ m $.\nThe ratio $ 5:4 = 20:m $, which translates mathematically to:\n$$\n\frac{5}{4} = \frac{20}{m}\n$$", "---", "### Step-by-Step Solution", "Start with the ratio equation:\n$$\n\frac{5}{4} = \frac{20}{m}\n$$", "To solve for $ m $, apply cross-multiplication:\n$$\n5 \ imes m = 4 \ imes 20\n$$", "Simplify the right-hand side:\n$$\n5m = 80\n$$", "Now divide both sides by 5 to isolate $ m $:\n$$\nm = \frac{80}{5} = 16\n$$", "---", "### Conclusion", "There are exactly $ \boxed{16} $ mackerel. This method—setting up the proportion, cross-multiplying, and solving—provides a clear and reliable way to tackle ratio queries in algebra and everyday problem-solving.", "Whether you're managing fish counts on a boat or analyzing data, this technique empowers quick and accurate calculations.", "---", "Keywords:\nmackerel ratio problem, solve ratio $5:4=20:m$, algebra ratio solution, how to solve ratio equations, cross-multiplication example, mackerel calculation, solve for $m$"]