The riddle
If It Took 3 Cooks 4 Hours to Roast a Turkey, How Long Would It Take 4 Cooks to Roast the Same Turkey?
The trap in the numbers
Three cooks working four hours add up to 12 cook-hours of work. Spread across four cooks, that is 3 hours each. A listener who sets up the sum and gets 3 has fallen into the trap. The trick is not in the arithmetic at all. It sits in two words near the end of the question.
Same turkey
The question asks about roasting "the same turkey". That turkey is the one the three cooks already roasted in four hours. A turkey that has already been roasted needs no more roasting, so the time needed is none. The answer is not that the job is impossible. It is that the job is finished before the question begins.
Why the sum would not work anyway
Wikipedia's article on the man-hour defines it as the amount of work done by the average worker in one hour. It adds that some tasks cannot be divided: a chicken roasts no faster with two cooks, and a second chef halves chopping time but not carving time. Heat does the roasting, and extra people cannot add to it. The article also mentions Fred Brooks, who wrote in The Mythical Man-Month that the cost of organizing and coordinating extra people can outweigh their benefit, especially over a short period.
Setting the trap in a group
Give the question to a group and have everyone work out the answer on paper before anyone speaks. Collect the answers, then read the question aloud once more, stressing the last three words. A related number trick is how many times can you subtract 5 from 25, where one word in the question changes the answer.