alleryWhy is 5 prime but 4 is not?
A5 is an odd number, but 4 is an even one
B5 factors only as 1 x 5, but 4 = 2 x 2
C5 comes after 4, so 5 must be prime
D5 is the sum of 2 and 3, but 4 is not
Answer & Solution
Correct answer: B. 5 factors only as 1 x 5, but 4 = 2 x 2
1. The only ways to write 5 as a product are 1 x 5 or 5 x 1, which involve 5 itself.
2. 4 is a product of two smaller numbers: 2 x 2.
3. Being odd does not make a number prime (9 is odd and composite).
_Source: Wikipedia, Prime number, section Definition and examples_
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