Respuesta :

[tex]\begin{gathered} l=\text{prt} \\ l(\frac{1}{p}\frac{1}{r})=(\frac{1}{p}\frac{1}{r})\text{prt} \\ t=\frac{l}{pr} \end{gathered}[/tex]

Then

[tex]t=\frac{l}{pr}[/tex]