According to the order of operations (often remembered by the acronym PEMDAS: Parentheses, Exponents, Multiplication and Division (from left to right), Addition and Subtraction (from left to right)), we first look for any operations inside parentheses.
In the expression \([15 + (-5)] + 6 / 2\), the first simplification takes place inside the parentheses:
\[ 15 + (-5) = 10 \]
Thus, the first simplification is \(15 + (-5)\).