¿How many sexy primes are between 3! and 4!?
Number Theory
Level
2
Sexy prime numbers are prime numbers that differ exactly by 6
n! is (n-1) (n-2) (n-3)...
17
Enough
67
5
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3 ! = 6 and 4 ! = 2 4 . There are 2 4 − 6 = 1 8 numbers between 2 4 and 6 . Half of these numbers are odd, so there are 9 odd numbers between 2 4 and 6 . We don't even need to check which numbers are prime, because there is only one answer less than 9 .