Explanation

r is inversely proportional to p³, so r = k/p³.

Given p = 9 and r = 2:

2 = k/729

k = 2 × 729 = 1458

Now, r = 1/4:

1/4 = 1458/p³

p³ = 1458 × 4 = 5832

p = 18