Question

three box = 30 with these digits 1,3,5,7,9,11,13,15

Answer 1

it is not possible to obtain even number by adding odd number
by fectorial method you can get the answer
3! + 11 + 13 = 30 
3! + 9 + 15 = 30
3! = 1*2*3 = 6

Answer 2

There can be many answers but the basic idea is using factorial. Factorial is the product of all the numbers from 1 to that number. For example 4! = 1 x 2 x 3 x 4 = 24.
 
So, 3! + 11 + 13  = 30
Similarly you can find many such answers.

Hope that helps !!