OEIn this problem, each of the digits 1–5 can be either the tens digit, the units digit, or not a digit in the jersey number. What you’re really counting is the number of unique jersey numbers.
Make two slots, one for the tens digit and one for the units digit. You have 5 choices for the tens digit and 4 choices for the units digit (since you cannot use the same digit again), resulting in 5 × 4 = 20 possibilities. The slot labels are different (Tens and Units), so don’t divide 20 by anything. You could also list out the jersey numbers, since the number of possibilities is relatively limited.

There are 5 groups of 4 each for 20 total possibilities. If you notice partway through your list that each of the 5 possible tens digits will have 4 possibilities (any of the available units digits other than the same), use the pattern to solve more quickly.
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