set theory

Question

How many minimum number must be chosen from a set {1,2,3,...8} such that at least 2 of them must have sum=9?

Comments

5 numbers.!

Answer 1

Let us see which pair can give you 9

(1,8),(2,7)(3,6),(4,5)

So let us take 2 set

A={1,2,3,5,X} B={8,7,6,5,Y}

Now since repetition is not allowed this for any value of X or Y there will be a pair which will give you 9