Permutation...

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Can you help me with this? We, me and classmates are trying to solve this problems but still don't know how to answer this kind of question. Well the teacher gave the answer but still don't gave us on how to solve this problem. Can you help us answer this problem. Those who are good in permutation.. Please help us.. Here it is.. 1. How many 3-digit numbers can be constructed from the digit 0, 7, 3, 4, 6, 9, and 5 if a) there are no restrictions? Answer: 294 b) the number must be greater than 400 and repetition is not allowed? Answer: 150 c) the number must be odd and repetition is not allowed? Answer: 50 d) the number must be divisible by 5 and repetition is not allowed? Answer: 55 Can you give me the solution.. Thanks...