Neutron stars spin very fast, lets fid in what time it completes on revolution.

$Formula P_{ns} = P_c(\frac{R_{ns}}{R_c})^2$

$P_{ns} = period of neutron star$

$P_c = period of core$

$R_{ns} = radius of neutron star$

$R_c = Radius of core$

Ans::

$Formula P_{ns} = P_c(\frac{R_{ns}}{R_c})^2$

$P_{ns} = period of neutron star$

$P_c = period of core$

$R_{ns} = radius of neutron star$

$R_c = Radius of core$

Ans::

So the radius of the core will be taken as 3Km and radius of neutron star is 17Km, period of core is say 0.000007seconds

$ P_{ns} = 0.0007(\frac{17}{3})^2 = 0.000335sec$

now

$R_c = 0.7Km, R_{ns} = 20Km, P_c=3.6e-6sec$

so

$ P_{ns} = 3.6e-6(\frac{20}{0.7})^2 = 0.0029sec$

Now I exagerate:

$R_{ns} = 15.5Km, R_c = 0.06Km, P_c = 0.000000006sec$

so:

$P_{ns} = 0.000000006(\frac{15.5}{0.06})^2 = 0.0004sec$

$P_{ns} = 0.000000006(\frac{15.5}{0.06})^2 = 0.0004sec$