From the set of numbers s={1, 2, 3, 4}, how many minimum numbers must be selected to guarantee that at least one pair of these numbers has a sum equal to 7?
How to get the answer of this question
You need both 3 and 4 to be picked. If any is not picked you can’t make pair with 7. So you need to pick all numbers
Refer this for another such example https://www.quora.com/What-is-the-minimum-number-of-integers-to-be-selected-from-the-set-1-2-3-11-to-ensure-that-some-two-s-of-these-numbers-sum-to-12
It is a simple question which states that find the number of doublets (pairs) you have to pick so that you are sure that atleast one of the pairs form a sum 7. If you dont have prior knowledge to piegon hole principle I would recommend you to have a look at this.
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