If I understand the problem correctly then the smallest angle cannot be 115, could you mean 11.5 instead?
If so then you basically have 18 angles starting at 11.5 and increasing by a value n. All angles total 360 therefore:
11.5 + (11.5 + n) + (11.5 + 2n) + ... + (11.5 + 17n) = 360
n works out nicely actually if 11.5 was what you meant. Can you solve it from here or do you need more help?