-
Target Test Prep 20% Off Flash Sale is on! Code: FLASH20
Redeem
Patterns in Divisibility Problems
Today were going to tackle a couple of tough divisibility problems from GMATPrep. The two problems Ive chosen share some interesting characteristics. Heres your first one; set your timer for 2 minutes and go!
If n is a multiple of 5 and n = [pmath]p^2[/pmath]q, where p and q are prime numbers, which of the following must be a multiple of 25?(A) [pmath]p^2[/pmath]
(B) [pmath]q^2[/pmath]
(C) pq
(D)[pmath]p^2*q^2[/pmath]
(E) [pmath]p^3[/pmath]q
Hmm. So p and q are primes. They could be 2, 3, 5, 7 or so on. It doesnt say that p and q are different prime numbers, so they could also be the same number. And Im going to use some theory here: if[pmath]p^2[/pmath]q equals n and n is a multiple of 5, then that 5 must be "contained in" either p or q. And since those two numbers are primes, either p or q is 5. (Or maybe both are!)
How did I figure that out? Look at this (totally different) example:
56 = 8 7
56 is a multiple of 4. Do either of the two numbers on the right-hand side contain a 4? Yes, 8 contains a 4.
Come up with an equation like the one above in which the number on the left is a multiple of some number, but you CANNOT find that number in one of the numbers on the right.
56 = 7 <something that does NOT contain a 4>
We cant do it. If the two sides are equal, they must simplify down to the same primes:
56 = 2 2 2 7
8 7 = 2 2 2 7
Okay, so thats how I know that if the n, on the left side, contains a 5 as a factor, then the [pmath]p^2[/pmath]q on the right side also has to contain a 5 somewhere. Since we were told that p and q are both primes, we know that at least one is equal to 5.
Which one? We have no idea. Now, the problem is asking us how to get 25. Well, if p is 5, then wed need[pmath]p^2[/pmath] to get 25. And if q is 5, then wed need[pmath]q^2[/pmath] to get 25.
So how can we guarantee that we get 25? Have both of those! With[pmath]p^2*q^2[/pmath], it doesnt matter whether the p is 5 or the q is 5 either way, we square whichever one is 5 to get 25!
The correct answer is D.
The key takeaway here is this: when were told two expressions equal each other, then those two expressions must simplify down to the same pieces (on each side of the equation). Remember that.
Ready for the next one? You know the drill (2 minutes!).
If n and y are positive integers and 450y =[pmath]n^3[/pmath], which of the following must be an integer?I. [pmath]y/{(3)(2^2)(5)}[/pmath]
II. [pmath]y/{(3^2)(2)(5)}[/pmath]
III. [pmath]y/{(3)(2)(5^2)}[/pmath]
(A) None
(B) I only
(C) II only
(D) III only
(E) I, II, and III
Oh, yay. A roman numeral question. (Yes, thats sarcasm.)
Were told that we have positive integers and theres an equation. Now, whats the question asking? Which of the following must be an integer okay, and then each of the roman numerals contains y divided by some primes oh, I get it! If doing the division results in an integer, then that means y is divisible by those primes. This is actually a divisibility question, too, even though they dont say that word in the problem! Also, this is a MUST be true question, just like the last one.
So, what do we do with the equation, again, on divisibility questions? Ah yes that means the left-hand side must ultimately break down to the same thing on the right-hand side. Lets see if that helps us at all on this one.
450 = 45*10 = 3 3 5 2 5
[pmath]n^3[/pmath] = n n n
Hmm. So, heres what weve got (Im going to rearrange the numbers that make up 450 to put them in order):
2 3 3 5 5 y = n n n
Now what? Hmm, whats the significance of having three ns on the right-hand side? Each n is the same number and the ns on the right have to contain all of the same pieces as the stuff on the left
Thats interesting. Each n has to contain the exact same thing, and I cant break primes down any further. So if theres one 2 on the left, there actually has to be at least three 2s, because each n needs to get its own 2.
Too theoretical? Try a simpler, real-numbers-only example.
2 2 2 5 5 5 = b b b
What is the value of just one single b? We just split each set of 3 numbers on the right one 2 for each n and one 5 for each n, or n = 2 5.
In our harder problem, with the y, we dont have all of the numbers on the left because weve got this variable y. So we basically have to infer that, if theres one 2, there must be at least three 2s (so that each n can have its own 2). By the same token, if there are two 3s, there actually must be at least three 3s, and ditto for the 5s.
Where are those other numbers? They have to be part of the y variable. So the y has to contain at least two 2s, one 3, and one 5. (Technically, it could contain even more maybe it contains three 17s as well! But the question asks us what must be true, so we dont have to speculate about what else we might have.)
If the y must contain two 2s, one 3, and one 5, then how can we use that knowledge to evaluate the three roman numerals? First, lets represent y mathematically:
y = 2 2 3 5 ?
We know we have those 4 numbers plus there might be other stuff, so well include the question mark, just in case.
If I take that and plug it into the first roman numeral, am I going to get an integer?
Lets see:
I. [pmath]{2*2*3*5*?}/{(3)(2^2)(5)}[/pmath]
Start crossing stuff off. Hey, I can definitely eliminate everything on the bottom! That means I dont have anything left on the denominator except for the number 1, so this must simplify to an integer. Yay! Roman numeral 1 works.
Eliminate answers A, C, and D. Excellent! Notice that we only have to test one more roman numeral, either II or III. Thatll be enough to get the answer, because it can only be either B or E at this point. If one of those roman numerals looks a lot easier to you than the other, make sure to do the easier one next. (In my opinion, these two look pretty similar, but sometimes there is a big difference in difficulty level.)
Lets try roman numeral II.
II. [pmath]{2*2*3*5*?}/{(3^2)(2)(5)}[/pmath]
Hmm. I can cross off the 2 and the 5. I can also cross off one of the 3s. Can I cross off the other 3? Maybe maybe the ? contains another 3 but I dont know for sure. So theres a possibility that Im left with a 3 on the bottom of this fraction which means I wont get an integer. This one isnt a MUST be true statement, so I can eliminate answer E.
The correct answer is B.
Lets look at roman numeral III to be thorough (though on the real test, dont do more work than you have to do!).
III. [pmath]{2*2*3*5*?}/{(3)(2)(5^2)}[/pmath]
This one is like roman numeral II. I know I can definitely divide out the 2, the 3, and one of the 5's on the denominator, but I dont know that I can definitely divide out both 5s. This one is also not a MUST be true statement.
Key Takeaways for Divisibility Problems:
(1) If the problem is about divisibility and were given an equation, then were probably going to need to break both sides of the equations down to the basic components and compare.
(2) A divisibility problem might not use normal divisibility language (divisible, multiple, factor, ). For example, it may just ask us whether we get an integer for some math setup that includes division we have to recognize that they really are asking about divisibility and factors.
(3) On MUST be true questions, we have to find something that is always true, not just sometimes true. There will be traps revolving around things that could be true but dont absolutely have to be true watch out for those traps!
* GMATPrep questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.
Recent Articles
Archive
- April 2024
- March 2024
- February 2024
- January 2024
- December 2023
- November 2023
- October 2023
- September 2023
- July 2023
- June 2023
- May 2023
- April 2023
- March 2023
- February 2023
- January 2023
- December 2022
- November 2022
- October 2022
- September 2022
- August 2022
- July 2022
- June 2022
- May 2022
- April 2022
- March 2022
- February 2022
- January 2022
- December 2021
- November 2021
- October 2021
- September 2021
- August 2021
- July 2021
- June 2021
- May 2021
- April 2021
- March 2021
- February 2021
- January 2021
- December 2020
- November 2020
- October 2020
- September 2020
- August 2020
- July 2020
- June 2020
- May 2020
- April 2020
- March 2020
- February 2020
- January 2020
- December 2019
- November 2019
- October 2019
- September 2019
- August 2019
- July 2019
- June 2019
- May 2019
- April 2019
- March 2019
- February 2019
- January 2019
- December 2018
- November 2018
- October 2018
- September 2018
- August 2018
- July 2018
- June 2018
- May 2018
- April 2018
- March 2018
- February 2018
- January 2018
- December 2017
- November 2017
- October 2017
- September 2017
- August 2017
- July 2017
- June 2017
- May 2017
- April 2017
- March 2017
- February 2017
- January 2017
- December 2016
- November 2016
- October 2016
- September 2016
- August 2016
- July 2016
- June 2016
- May 2016
- April 2016
- March 2016
- February 2016
- January 2016
- December 2015
- November 2015
- October 2015
- September 2015
- August 2015
- July 2015
- June 2015
- May 2015
- April 2015
- March 2015
- February 2015
- January 2015
- December 2014
- November 2014
- October 2014
- September 2014
- August 2014
- July 2014
- June 2014
- May 2014
- April 2014
- March 2014
- February 2014
- January 2014
- December 2013
- November 2013
- October 2013
- September 2013
- August 2013
- July 2013
- June 2013
- May 2013
- April 2013
- March 2013
- February 2013
- January 2013
- December 2012
- November 2012
- October 2012
- September 2012
- August 2012
- July 2012
- June 2012
- May 2012
- April 2012
- March 2012
- February 2012
- January 2012
- December 2011
- November 2011
- October 2011
- September 2011
- August 2011
- July 2011
- June 2011
- May 2011
- April 2011
- March 2011
- February 2011
- January 2011
- December 2010
- November 2010
- October 2010
- September 2010
- August 2010
- July 2010
- June 2010
- May 2010
- April 2010
- March 2010
- February 2010
- January 2010
- December 2009
- November 2009
- October 2009
- September 2009
- August 2009