Composite primes

[poonam1279]

Need help in solving this

[cramya]

Take the easier of the 2 statements:

Stmt II

2 digit number greater than 20 with tens digit 2


23 - not composite
24 - composite

INSUFF

Stmt I

Numbers

22,24,26,28,30,33,36,39,40,,44,48,50,55,60,66,70,77,80,88,90,99

All of these are composite

SUFF

A)

Other than listing the numbers which can be done within 2 minutes I coudnt think of a shorcut

[logitech]

Other than listing the numbers which can be done within 2 minutes I coudnt think of a shorcut

You don't need a shortcut here bro. Good thinking , clean solution! Sweet!

[cramya]

Keep in mind the following:

1) 0 is a multiple of every number
2) 1 is a factor of every number
3) 0 has infinetely many factors (thats why I added 30,40,50,60,... to the list above since the tens digit in each number is a factor of units digit)

[pbanavara]

From 1: Since the Tens has to be a factor of the Units - The number has to have a factor which is not 1 and itself. Hence it can't be prime. Trial and error - but took less than 2 mins

- pradeep

[earth@work]

Keep in mind the following:

1) 0 is a multiple of every number
2) 1 is a factor of every number
3) 0 has infinetely many factors (thats why I added 30,40,50,60,... to the list above since the tens digit in each number is a factor of units digit)

cool! "1 is a factor of every number" just slipped my mind while solving this problem.

[vishubn]

Cramya: 23 is not composite !! but arent we supposed t oconsider only numbers which are composite ! and wit hthis how can u proce D is Insuff!

I am sure i am askign somethign simple ! but i dont get it !! sorry

VIshu

[cramya]

Cramya: 23 is not composite !! but arent we supposed t oconsider only numbers which are composite ! and wit hthis how can u proce D is Insuff!

I am sure i am askign somethign simple ! but i dont get it !! sorry

No probs Vishu!

U r correct. . 23 is prime and not composite.

Stmt II says 2 digit number greater than 20 with tens digit of 2

If its 23 then its prime
If its 24 then its composite

INSUFF since 2 different answers (hence D) not possible)


In the problem they have given the definition of a composite number which is a bonus and they are not telling us that we need to consider only composites.

Hope this helps!

[vishubn]

In the problem they have given the definition of a composite number which is a bonus and they are not telling us that we need to consider only composites.

More than the solution , the statement helped thanks cramya ! i am sure i am all konked

thanks again

Vishu