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  is given as 

$B=\frac{-pV}{\Delta V} ....\left(i\right)$

The volume of a spherical object of radius  r  is given as 

$V=\frac{4}{3}\pi r^3, \Delta V=\frac{4}{3}\pi \left(3r^2\right)\Delta r$

$\therefore \frac{V}{\Delta V}=\frac{\frac{4}{3}\pi r^3}{\frac{4}{3}\pi 3r^2\Delta r}\text{ or }\frac{V}{\Delta V}=\frac{r}{3\Delta r}$

Put this value in eqn. (i), we get 

$B=\frac{pr}{3\Delta r}$

$\text{ Fractional decrease in radius is }$

$-\frac{\Delta r}{r}=\frac{p}{3B}$