What's the answer of (10+13) ≡? As to Modulus operation I only have seen this form:
(x + y) mod z  ≡ K

So I can't understand the question, by the way the answers are :
a) 8 (mod 12)
b) 9 (mod 12)
c) 10 (mod 12)
d) 11 (mod 12)
e) None of the above


It is d). 
$23 \equiv 11 \text{ mod } 12$ since $(12)(1) + 11 = 23$. In other word, the remainder upon division by $12$ is $11$. 
