Chapter 1 - Introduction to Algebraic Expressions - 1.2 The Commutative, Associative, and Distributive Laws - 1.2 Exercise Set - Page 17: 67

$5r+10+15t$

The distributive law states that $a(b+c)=ab+ac$ (for real numbers $a$, $b$, and $c$). That is, the product of a number and a sum can be written as two products. Therefore, $5(r+2+3t)=5(r)+5(2)+5(3t)=5r+10+15t$.