Divide the numbers from 1,2,3,4,5,6,7,8 as groups
like (1,8) (2,7) (3,6) (4,5)
Now we have four groups
Suppose take 4 numbers from 4 groups like 1,2,3,4 but some not equal to 9.
If we choose 5 numbers 1,2,3,4,8 or 1,2,3,4,7 then sum is 9.
Thus a minimum of 5 numbers are needed.
