Lesson: Prime Numbers

Comment on Prime Numbers

Hi Brent,

Do you have the video slides for this lesson available for download? I know you have some available in linkedin.

Thanks!
gmat-admin's picture

Hi bcc123,

We don't have slides for each individual video lesson. However, we do have slides that cover all of GMAT math here: https://www.slideshare.net/GMATPrepNow_free/gmat-math-flashcards and here: https://gmatclub.com/forum/new-resource-interactive-flashcards-for-gmat-...

sir doubt
https://gmatclub.com/forum/if-x-y-y-1-and-y-is-a-prime-number-less-than-11-which-of-the-f-223453.html
gmat-admin's picture

Happy to help!

My step-by-step solution can be found here: https://gmatclub.com/forum/if-x-y-y-1-and-y-is-a-prime-number-less-than-...

Cheers,
Brent

Is the positive integer x even?

(1) (x - 1) is a prime number
(2) (x^2 - 1) is a prime number

statement 1 = if x = 3 then x-1 is prime
if x = 6 then x-1 is prime
so insuff
statement 2
x^2-1 = prime
only 2 satisfies this
so suff
is this approach correct???
gmat-admin's picture

Perfect approach!

Cheers,
Brent

Hi Brent, could you please help me with the below question

https://gmatclub.com/forum/how-many-positive-integers-less-than-30-are-either-a-multiple-of-2-an-127362.html
gmat-admin's picture

Hi Brent here's another one I need your help with

https://gmatclub.com/forum/set-s-consists-of-more-than-two-integers-are-all-the-numbers-in-set-s-152717.html

https://gmatclub.com/forum/if-p-is-a-positive-integer-is-p-a-prime-number-172237.html
sir in statement 1 how to know which values to test?
gmat-admin's picture

Question link: https://gmatclub.com/forum/if-p-is-a-positive-integer-is-p-a-prime-numbe...

In this question, we're asked whether p is a prime number.
The GMAT loves to test whether students are aware that 2 is a prime number (in fact 2 is the ONLY prime number that's even). In fact, 2, 3 are the ONLY two consecutive primes.
So, that's why I tested p = 2

Notice that 2 and 3 (aka, p and p+1) have TWO factors each (making them both prime)
So, at that point, I started looking for 2 consecutive integers that each have FOUR factors each, which would make those values NOT prime numbers (aka composite numbers).

Do 3 & 4 work? No, 3 has two factors, and 4 has three factors.
Do 4 & 5 work? No, 4 has three factors, and 5 has two factors.
Do 5 & 6 work? No, 5 has two factors, and 6 has four factors.
.
.
.
Do 14 & 15 work? YES! 14 has FOUR factors, and 15 has FOUR factors.

Cheers,
Brent

https://gmatclub.com/forum/how-many-positive-integers-less-than-30-are-either-a-multiple-of-2-an-127362.html

I'm not sure if the approach I used is right:

Number of multiples of 2 between 2 and 30:

2(1) ... 2(15) 15 - 1 + 1 = 15
Take away 1 as its integers below 30
So total is 14

Number of odd prime numbers below 30:

9

Sum of positive multiples of 2 and odd prime numbers:

2 + 19 = 21
2 + 23 = 25
4 + 23 = 27

Any other calculations would have resulted in overlap and repetition. So total numbers that satisfied the statement was: 14+9+3 = 26

Where am I going wrong and how could this have been done much much more simpler in under 2 mins?
gmat-admin's picture

In your approach, what's the rationale behind choosing the smallest odd prime 3 as opposed to 5 or 7?
gmat-admin's picture

Choosing 3 ensures that I don't miss any values.

That said, in this particular example, I could have chosen 5 or 7 and still reached the correct answer, but it's still best to start with the smallest odd prime.

For example, if I had chosen 11, then I would have missed the opportunity to get 9 as one of the possible values.

Cheers,
Brent

https://gmatclub.com/forum/how-many-positive-integers-less-than-30-are-either-a-multiple-of-2-an-127362.html
i did not get the 30 second approach given by Bunuel
gmat-admin's picture

Question link: https://gmatclub.com/forum/how-many-positive-integers-less-than-30-are-e...

Bunuel's solution is pretty much the same as my 30-second solution (at https://gmatclub.com/forum/how-many-positive-integers-less-than-30-are-e...). The main difference is that Bunuel uses fewer words :-)

Cheers,
Brent

https://gmatclub.com/forum/if-the-integer-n-is-greater-than-1-is-n-equal-to-94720.html

would you please give your approch?
gmat-admin's picture

The product of all the prime numbers less than 20 is closest to which of the following powers of 10 ?
(A) 10^9
(B) 10^8
(C) 10^7
(D) 10^6
(E) 10^5
The answer is D right?

Prime numbers are 2,3,5,7,11,13,17,19.

To approximate the product of those prime numbers we can do the following:
2*5=10
3*7=20
11*19=200
13*17=200

Now lets multiply 10*20*200*200=8000000 and according to scientific notation the result should be written as 8*10^6.

If to take calculator and make a precise calculation the answer is also 10^6. Do I understand it correctly? Please help me out. Thank you.
gmat-admin's picture

Your approach is great.

However, you need to recognize that your approximations are all a bit smaller than than the actual products.
For example, 11 x 19 = 209 (not 200), and 13 x 17 = 221 (not 200) etc.
So, your product of 8 x 10^6 is a bit LESS THAN the actual answer.
If we round UP, we get: 8 x 10^6 ≈ 10 x 10^6 ≈ 10^7

Here's my full solution: https://gmatclub.com/forum/the-product-of-all-the-prime-numbers-less-tha...

Does that help?

Cheers,
Brent

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