Question

_ + _ + _ + = 30 ,by using 1,3,5,7,9,11,13,15

Answer 1

sum of odd number of odd numbers will give you an odd number not even  ie 3 numbers out the given odd numbers will be an odd number only.

Answer 2

See according to the rules & naturality of maths sum of odd numbers in odd times will be an odd number like you taking about 3 odd numbers to be added & the answer should be 30 so it is not possible because 30 is an even number.