Rules for Assigning Oxidation Numbers — Prelims Questions

What is the oxidation number of chromium in $K_2Cr_2O_7$?

What is the oxidation number of sulfur in $H_2SO_5$ (Caro's acid)?

What is the oxidation number of nitrogen in $NH_4NO_3$?

Which of the following compounds contains oxygen with an oxidation number of $+2$?

In which of the following compounds does hydrogen exhibit an oxidation number of $-1$?

What is the average oxidation number of bromine in $Br_3O_8$?