Question

dash+dash+dash=30 use only (1,3,5,7,9,11,13,15)

Answer 1

As we know that sum of 3 odd number can't be even, but if we can do any mathematical operation within the bracket then only it can be possible.
i.e.
(15-5) + (11-1) + (13-3 )= 30

Answer 2

sum of 3 odd numbers is never equal to the even digit. so in this case it is not possible for us to get answer but we can use the method of factorization.
factorization symbol is - !
and for your understanding factorials are done like this :
suppose the a number is given with factorial 3!
3! = 1*2*3 = 6
4! = 1*2*3*4 = 24
5! = 1*2*3*4*5 = 120
so factorial method is used like the above examples. 

so we can use factorial for this question -
 4! + 3! = 30
4! + 3 + 3 = 30