The value of the expression [9]+[11] in Z_{15} and to write the answer in the form [r] with 0\leq r < m.

Definition used:
Congruence.
Let m be an integer greater than 1. If x and y are integers, then x is congruent to y modulo m if x — y is divisible by m. It can be represented as ${\displaystyle x=y\pm odm}$. This relation is called as congruence modulo m.
Calculation:
It is known that, [x] + [y] = [x+y].
Then, the value of the expression becomes as follows.
[9] + [11] = [9 + 11] = [20]
Since ${\displaystyle 20\equiv 5\pm od15}$. Thus [9] + [11] = [5] in ${\displaystyle Z_{15}}$.
Therefore, the value of the expression is [5].