One approach is to rewrite each equation so that they both feature the same multiple of \(s\). Here's what I mean:

Take: \(3p = 4s\)
Create an equivalent equation by multiplying both sides by \(5\) to get: \(15p = 20s\)

Take: \(5s = \frac{3}{r}\)
Create an equivalent equation by multiplying both sides by \(4\) to get: \(20s = \frac{12}{r}\)

Since both equations are set equal to \(20s\), we can create the following equation: \(15p = \frac{12}{r}\)

Multiply both sides of the equation by \(r\) to get: \(15pr = 12\)

Divide both sides of the equation by \(15p\) to get: \(r = \frac{12}{15p}= \frac{4}{5p}\)

Answer: A