The bulk modulus of a spherical object is 'B'. If it is subjected to uniform pressure 'P', the fractional decrease in radius is:

Bulk modulus B = -V(ΔP)/(ΔV)

For a sphere, V = (4/3)πr³
ΔV = (4/3)π(3r²Δr) = 4πr²(Δr)

Substituting into the bulk modulus equation:
B = -((4/3)πr³)(P)/(4πr²Δr)
B = -rP/(3Δr)
Δr/r = -P/(3B)
Fractional decrease in radius = -Δr/r = P/(3B)