Degree of unsaturation
Degree of unsaturation is always a good way to guide the mind to find possible molecular formula.
The degree of unsaturation is the sum of the "pi-bond" and rings.
In this exercise you need to find the degree of unsaturation for various molecules.
P and S are at their lowest valence.\(d_U = {2 n_C + n_N - n_H - n_X \over 2} + 1\)