you reproduce the commercial at 20 frames every second, to find how many seconds do you need to reproduce 100 frames, we can use the next proportion:
[tex]\frac{20\text{ frames}}{100\text{ frames}}=\frac{1\text{ second}}{x\text{ seconds}}[/tex]Solving for x:
[tex]\begin{gathered} 20\cdot x=1\cdot100 \\ x=\frac{100}{20} \\ x=5\text{ seconds} \end{gathered}[/tex]