Answer: He spent 3 hours as a cashier and 6 hours as a lifeguard
Explanation:
First, you figure out his time as a cashier; 12($ per hour) x 3(hours working) = $36. Then, since it states he spent twice as many hours being a lifeguard, you double his hours when figuring out his time as a lifeguard; 18($ per hour) x 6(hours working) = $108. Add up both of your totals, $108 + $36 = $144.