Surfer #1 takes a capsule every 6 hours, but this is also equal to 2 capsules every 12 hours. Surfer #2 takes 1 capsule every 12 hours. Thus, both surfers take a total of 3 capsules every 12 hours.
By dividing 45 total capsules they need to consume by 3, the number of capsules the surfers consume every 12 hour, you get the total number of hours times 12 it takes for them to consume 45 capsules. 45 / 3 = 15, so it takes 15 * 12 hours for them to consume 45 capsules. This is equal to 180 total hours.
