To find the oxidation number we can check the periodic table of the elements. There are some elements that have only one oxidation state, others can have more than one.
We must take into account that the total oxidation state of the molecule must be 0, so the sum of the oxidation states of the elements must equal 0.
To start, let's identify the possible oxidation states of elements in molecules.
Pb: +4, +2
Cr: +6, +3, +2
Or: -2
Oxygen has only one oxidation state equal to -2 so we'll start by placing that. Regarding chromium, in this case, the only possibility is that it has an oxidation state of +6 since if the oxidation state is lower, the molecule could not be neutralized with a single Pb atom.
With an oxidation state of oxygen of -2, of Cr equal to +6, the lead must have an oxidation state of +4. To understand it we can see the following table:
Therefore, the answer will be:
p) Oxidation number of Pb in Pb(Cr2O7)2: +4
q) Oxidation number of Cr in Pb(Cr2O7)2: +6
r) Oxidation number of O in Pb(Cr2O7)2: -2