Question

-- + -- + -- = 30....you can only use 1,3,5,7,9,11,13,15

Answer 1

The riddle can around various platforms - quora and whatsapp to name a few. There are a lot of answers to it.  

1. The most obvious one - 3 odd numbers added together can never form an even number. i.e. 30 in this case. Hence, the question is wrong.

2. Then there are people who are using decimals to solve this. For example -
13.3+15.7+1=30

3. An interviewer asked me this question and this was his answer. (Using other mathematical expressions) - 3! + 11 + 13 = 30

Conclusion - The question needs to be more specific. If you play around with these numbers, there will be infinite number of ways to solve this. 
Else this question is merely a stupidity test. (back to Point 1)
And hence, it is popular.