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If p and q are positive integers, how many integers are lar [#permalink]
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02 Dec 2013, 19:35
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If p and q are positive integers, how many integers are larger than pq and smaller than p(q + 2)? A. 3 B. P + 2 C. p – 2 D. 2p – 1 E. 2p + 1 Please solve using algebraic method because substitution will sometimes get the right answer but sometimes not.
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Re: If p and q are positive integers, how many integers are lar [#permalink]
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02 Dec 2013, 19:45
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If p and q are positive integers, how many integers are larger than pq and smaller than p(q + 2)?
First, we notice that: p(q+2) = pq + 2p
So we are asking how many integers are larger than pq but smaller than pq+2p. Notice that the range of this is 2p, so our answer should be around 2p. When you count the number of integers in between two other integers, you take high number  low number  1.
Answer: pq + 2p  pq  1 = 2p 1
To see why this counting formula works, try it out. How many numbers between 1 and 3? 311 = 1 How many numbers between 1 and 4? 411 = 2 2 and 2? 2(2)1=3
Last edited by jamesphw on 02 Dec 2013, 19:46, edited 1 time in total.



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Re: If p and q are positive integers, how many integers are lar [#permalink]
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02 Dec 2013, 19:46
smartyman wrote: Please help....
If p and q are positive integers, how many integers are larger than pq and smaller than p(q + 2)? A) 3 B) P + 2 C) p – 2 D) 2p – 1 E) 2p + 1
Please solve using algebraic method because substitution will sometimes get the right answer but sometimes not. The number of integers larger than pq and smaller than p(q+2) will be the number of integers between the two values. So, we start with the difference between the two: p(q+2)  pq pq + 2p pq 2p Then, subtract 1 for the upper bound (p(q+2). So, 2p1 is the correct answer.
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Re: If p and q are positive integers, how many integers are lar [#permalink]
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03 Dec 2013, 01:21
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If p and q are positive integers, how many integers are lar [#permalink]
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26 Dec 2014, 12:55
Hi smartyman, Did you try TESTing VALUES ("plugging in", "substitution" or whatever else you might choose to call it)? What happened when you did? When you mentioned that it "will sometimes get the right answer but sometimes not", I think that you mean to say that "in most cases it will get you the correct answer on the first try, but in some cases there might be more than one answer that comes back as a match, so you would have to do a bit more work and substitute a second time." TESTing VALUES is a remarkably fast and useful approach on many Quant questions on Test Day, so you shouldn't avoid that tactic outright. Here, if we use... P=3 Q=4 We have to figure out the number of integers that are greater than 12 and less than 18. THAT is not a very difficult task: 13, 14, 15, 16, 17 > a total of 5 terms. Plugging P into the answer choices gives us just one match: GMAT assassins aren't born, they're made, Rich
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Re: If p and q are positive integers, how many integers are lar [#permalink]
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05 Jan 2015, 04:33
This is how I did it:
We need numbers between the 2 given values.
So, I started like this: pq<x<p(q+2)
Then, I plugged in numbers: p=1  q=2 pq= 1*2=2 and p(q+2)=1(2+2)=4, so there is one number in between: 3.
p=1  q=1 pq=1*1=1 and p(q+2)= 1(1+2)=3, so there is one number in between: 2.
Finally, I looked at the answer options: A. I skipped it because according to my solution there is only 1 number in between and not 3 B. P + 2: using p=1, as the first plug in number we get 1+2=3, we don't like that, so move on. C. p – 2: same plug in, we get 12=1, not really possible to have minus number or numbers in between, so move on. D. 2p – 1: same plug in, we get 2*12=21=1, we like that, but checking E just in case, since there is still time. E. 2p + 1: same plug in, we get, 2*1+12+1=3, we don't like it.
Only D gave as a solution only 1 number in between. So, D must be correct.
Anything wrong wit this approac? Time wise it didn't take that long (less than 2 mins definitely).



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Re: If p and q are positive integers, how many integers are lar [#permalink]
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