For even integers i.e.,( 2,4 ) we have 3 per integer and for odd integers i.e.,( 1,3,5 ) we have 2 per integer… So the total is 2+3+2+3+2 = 12 but since 5 appears twice it is 12-1 = 11

First of all thank you for your response!

Now I understand where I made the mistake. Because in a set, there cannot be more than of an element, so there cannot be two 5s.

And because the instances (1,4) and (2,1) both results in 5, one of them has to be eleminated.

Therefore, I have the following question. What strategy could I have employed here? I followed the logical path and recognized the pattern of 2,3,2,3,2 and summed all of them to get 12. But even that got me the wrong answer. I could have calculated all of the individual instances to get the correct answer, but that could have lost me lose some time. When can I know to spend extra time on a question, and when to stint the time on other questions?