Answer
$T=0.9s$
Work Step by Step
We know that
$T=2\pi\sqrt\frac{m}{K}$............eq(1)
Also,
$w=mg$
This can be rearranged and solved as:
$m=\frac{w}{g}=\frac{20}{9.8}=2.04$ kg
We plug in the known values in eq(1) to obtain:
$T=2\pi\sqrt\frac{2.04}{100}=0.9s$
