This is because the diagram represented isotopes of Nitrogen. Isotopes are forms of an element with an equal number of protons but different number of neutrons.
To solve this problem we will proceed to define the Period of a stick, then we will define the frequency, which is the inverse of the period. We will compare the change suffered by the new length and replace that value. The Time period of meter stick is

Here,
L = Length
g = Gravity
At the same time the frequency is

Therefore the frequency in Terms of the Period is

If bottom third were cut off then the new length is

Replacing this value at the new frequency we have that,


Finally,

Because of its weight if its heavy than its going to bounce low when you drop it but if it is not heavy than its going to bounce a little higher than the heavy ball