Chapter 2 - Equations, Inequalities, and Problem Solving - 2.4 Applications with Percent - 2.4 Exercise Set - Page 110: 78

Rik needs to make $\$ 21.60$ per hour.

For a comparable income, Rik needs to make $\$ 18+20\%\cdot\$ 18$. $ 20\%\cdot 18\qquad$....$ 20\%$ converted to decimal notation is $0.2$ $=0.2\cdot 18$ $=3.60$ Therefore, Rik needs to make $18+3.60=\$ 21.60$ per hour.