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.
You first need to take care that the valence of the nitrogen is 3 by adding charges !! Some exercises provide a valence of 5 which is wrong.