Discussion

How many four digit even numbers can be formed using the digits {2, 3, 5, 1, 7, 9}

Answer: D

A number is even only if its last digit is even, and 2 is the only even digit available.

\[ \text{last digit} = 1 \text{ choice} \]

The other three positions take the remaining five digits without repetition:

\[ 5 \times 4 \times 3 = 60 \]

\[ 1 \times 60 = 60 \]

Fixing the constrained position first is the standard tactic for these.

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