Discussion
Which one of the following is not a prime number?
Answer: D
Test each for divisibility.
\[ 91 = 7 \times 13 \]
31, 61 and 71 have no divisor other than 1 and themselves, so 91 is the composite one. It is a classic trap: it is odd, does not end in 5, and its digits sum to 10, so the easy tests all pass - only 7 catches it.
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