Answer Is B
The number 6! is a multiple of 3
The number 21 is also a multiple of 3
When two multiple of a certain number are added, the resultant number is also a multiple. Hence, 6!+21 has to be a multiple of 3 and can never be prime.
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prime no
Source: Beat The GMAT — Problem Solving |
Its just a quick method of doing sums related to primes;
All prime Nos above 5 can be expressed in the form 6k (+/-) 1
6! and 7! are both multiples of 6.
Just see if the Nos can be written in the from 6K +/-1.
1) is of the from 6k-1
2) is not
3) is of the from 6k-1
4) is of the from 6k-1
5)is of the from 6k-1
So its B IMO
Note: Not all Nos of the form 6k +/- are prime, but all primes can be expressed in that from (above 5)
In this case there was only 1, so we could directly say
All prime Nos above 5 can be expressed in the form 6k (+/-) 1
6! and 7! are both multiples of 6.
Just see if the Nos can be written in the from 6K +/-1.
1) is of the from 6k-1
2) is not
3) is of the from 6k-1
4) is of the from 6k-1
5)is of the from 6k-1
So its B IMO
Note: Not all Nos of the form 6k +/- are prime, but all primes can be expressed in that from (above 5)
In this case there was only 1, so we could directly say
mehrasa wrote:which one can not be prime?
1) 6!-1
2) 6!+21
3) 6!+41
4) 7!-1
5) 7!+11
OA after discussion
