Think of the requirements of this task in terms of "laborer-hours." In this case, if 6 laborers work at it for 8 hours, that's a total of 48 laborer-hours. By that reasoning, we could have 48 laborers work on it for one hour, or one laborer work on it for 48 hours; either way, we get to 48 laborer-hours, and the work gets done.

In this case, the goal is to get the task completed in 3 hours. Since it takes 48 laborer-hours, the number of laborers required is 48/3 = 16. We're looking for the number of additional laborers; since we already have 6, the number of additional laborers required is 16 - 6 = 10, choice (C).