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This is a basic problem on Combinations and Permutations. Here are 7 letters: A, L, T, E, M, P, N.
How many COMBINATIONS can you make from these letters (they don’t have to spell actual words) ?
How many PERMUTATIONS can you make ? (What is the longest actual word you can make from these letters?)
Would Combinations or Permutations typically be a larger number? Can you give an example with letters or numbers where both would be the same?
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There are many useful lessons on Permutations and Combinations in this site.
Two introductory lessons are
Introduction to Permutations and
Introduction to Combinations.
You will find there the full list of lessons, too, for further reading.