After solving look at the Complete Solution given


#1

Complete solution given is quite good.


#2

Yeah! No one can guess immediately this type of approach given in the complete solution.
We have 31, 28, 31, 30, ?.
What I did: 31 - 3 --> 28 + 3 --> 31
So, 31 - 1 --> 30 + 1 --> 31 and luckily I got answer!