SOLUTION: consider the numbers 2, 3, 5, 6 and 7. how many two digit numbers can be formed from these numbers if a. Repetition is not allowed b. repetition is allowed

There are 5 ways to choose the first digit.  

And for each of those 5 ways to choose the first digit,
there are 4 ways to choose the second digit.  So
that's 5×4 or 20 two digit numbers.  All 20 are listed
below:

23, 25, 26, 27, 
32, 35, 36, 37,
52, 53, 56, 57,
62, 63, 65, 67,
72, 73, 75, 76. 

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There are 5 ways to choose the first digit.  

And for each of those 5 ways to choose the first digit,
there are 5 ways to choose the second digit.  So
that's 5×5 or 25 two digit numbers.  All 25 are listed
below:

22, 23, 25, 26, 27, 
32, 33, 35, 36, 37,
52, 53, 55, 56, 57,
62, 63, 65, 66, 67,
72, 73, 75, 76, 77.

Edwin