y is inversely proportional to x if

y = k/x, where k is constant

the volume is 30 cubic centimeters at a pressure of 9 pounds means

30 = k/9

30*9 = k

k = 30*9

k = 270

let p represent the pressure when the volume is 60 cubic centimeters:

p = k/60

60p = k

plug k = 270 we have:

60p = 270

by solving we find:

p = 4.5 pounds

click here to see the step by step solution of the equation:

when the volume is 60 cubic centimeters the pressure is 4.5 pounds.