A mother bird has 6 babies.Everytime she returns to the nest,she feeds half the babies.If it takes her 5 mins to feed each baby,how long will it take her to feed all the babies?Ans:5 Hrs.
(manhattan word translations permutation/combination chapter #12)

My ques is that are'nt the babies distinct?Ofcourse thay are!The order DOES matter so this becomes a permutation problem.So if the mother feeds baby ABC or BCA or CAB does matter as the babies are not all AAA.So, the sol. becomes 6!/3!.If the babies were identical it becomes 6!/3!3!.

I was confused by the problem as you presented it, too. Please take care to post the exact wording of the problem, not a paraphrase. Here it is:

WT p. 80 #13
A mother bird has 6 babies. Every time she returns to the nest, she feed half the babies, even if some of those babies were fed the last time around. If it takes her 5 minutes to feed each baby, how long will it take her to feed all the possible combinations of the babies? Leave out the time the mother bird spends away from the nest.

If it were simply "how long will it take her to feed all the babies," the answer would be 6 * 5minutes = 30 minutes. (feed each baby once)

I have added italics for emphasis of the words that answer your question, I believe. The mother bird returns to the nest and feeds three different babies each time. Thus, "all the possible combinations of the babies" would treat ABC, ACB, BAC, BCA, CAB, and CBA as ONE distinct possible combination of babies that gets to eat in a single trip. Order does not matter, because A, B, and C all get to eat (i.e. they are the same in this situation. Thus, the 3 birds who don't eat in each trip are playing the same role, but likewise so are the 3 birds who do eat in each trip: 6!/3!3!.

On permutation questions, the "are the players different/distinct" question never revolves around whether the players are different individuals--the players are always different people, birds, chairs, etc. Instead, ask whether the players have the same role, experience, responsibility, etc. according to the story told in the question. In the case of the birds, the relevant question is "do birds A, B, and C all eat? Are they categorized the same as eaters, regardless of which one actually gets the worm first?" Since the answer is yes, order doesn't matter, and we divide by 3!.

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Emily Sledge
Instructor
ManhattanGMAT

Agree with you Emily. Mistook the stem "5 minutes to feed each baby", I simply multiplied 5 with 20 and got 100.... :(

This type of thing is great to catch in your review. Use moments like this not to beat yourself up but rather to highlight ways to improve your neatness/approach, so that you can avoid such errors in the future : )