Chapter 1 - Introduction to Algebraic Expressions - 1.2 The Commutative, Associative, and Distributive Laws - 1.2 Exercise Set - Page 17: 32

$m+(np+r)$

The associative law of addition states that $a+(b+c)=(a+b)+c$ (where $a$, $b$, and $c$ are real numbers). That is, changing the grouping of numbers to be added does not affect the answer. Therefore, $(m+np)+r=m+(np+r)$.