If m and n are positive integers and mn = p + 1, is m + n = p ?
Both m and n are prime numbers.
p + 1 and m are both even.
How to deal with this problem - odd&even method OR algebraic method?
DS - Prime
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- karthikpandian19
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- eagleeye
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Hi karthikpandian19:
The correct answer should be E. I am not sure whether odd/even or algebra would be faster but I did it using numbers for contradiction. Let me explain:
We are given mn=p+1, and asked whether m+n=p.
1. Both m and n are prime numbers.
If m, n are 2,3 then 2*3 = (2+3) + 1, (so m+n=p)
If m, n are 3,5 then 3*5 does not equal (3+5) + 1 (so m+n does not equal p)
INSUFFICIENT
2. p+1 and m are both even.
If m is even, then mn=p+1 must be even, using the same example as above where m=2, n=3, we know that
for that one, m+n=p, but if m=4 and n=3, then 4*3 is not equal to (4+3)+1, so insufficient.
At this point let's check them together. where m is an even prime and n is also a prime.
We have already checked m=2, n=3 and we know that in that case m+n=p
Let's check another one. Let m=2, n=5. In that case 2*5 is not equal to (2+5)+1.
Hence together they are insufficient as well. Hence E
Let me know if this helps
The correct answer should be E. I am not sure whether odd/even or algebra would be faster but I did it using numbers for contradiction. Let me explain:
We are given mn=p+1, and asked whether m+n=p.
1. Both m and n are prime numbers.
If m, n are 2,3 then 2*3 = (2+3) + 1, (so m+n=p)
If m, n are 3,5 then 3*5 does not equal (3+5) + 1 (so m+n does not equal p)
INSUFFICIENT
2. p+1 and m are both even.
If m is even, then mn=p+1 must be even, using the same example as above where m=2, n=3, we know that
for that one, m+n=p, but if m=4 and n=3, then 4*3 is not equal to (4+3)+1, so insufficient.
At this point let's check them together. where m is an even prime and n is also a prime.
We have already checked m=2, n=3 and we know that in that case m+n=p
Let's check another one. Let m=2, n=5. In that case 2*5 is not equal to (2+5)+1.
Hence together they are insufficient as well. Hence E
Let me know if this helps
- GMATGuruNY
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It is given that mn = p + 1.karthikpandian19 wrote:If m and n are positive integers and mn = p + 1, is m + n = p ?
Both m and n are prime numbers.
p + 1 and m are both even.
How to deal with this problem - odd&even method OR algebraic method?
Thus, if m+n = p, then mn = m+n + 1.
In other words, then the PRODUCT of m and n is one more than the SUM of m and n.
Question rephrased: Is the product of m and n one more than their sum?
Statement 1: m and n are prime.
Statement 2: p+1 (which is equal to the product of m and n) and m are even.
If m=2 and n=3, both statements are satisfied.
Here, the answer is YES: the product (2*3=6) is one more than the sum (2+3=5).
If m=2 and n=5, both statements are satisfied.
Here, the answer is NO: the product (2*5=10) is not one more than the sum (2+5=7).
Since the answer can be YES or NO, the two statements combined are INSUFFICIENT.
The correct answer is E.
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My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
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I used algebra to solve this problem.
If m + n = p, then mn = (m+n) + 1
mn - m = n + 1
m(n - 1) = n + 1
m = (n + 1) / (n - 1)
At this point, I realized that n = 2 or n = 3. Otherwise, m will not be a positive integer.
So, (m,n) = (3,2) or (2,3)
Statement 1: Both m and n are prime numbers.
(m,n) = (2,3) satisfies the equation above.
(m,n) = (2,5) doesn't satisfy the equation.
Statement 2: p + 1 and m are both even.
(m,n) = (2,3) satisfies the equation above.
(m,n) = (2,5) doesn't.
Statement 1 & 2:
(m,n) = (2,3) satisfies the equation.
(m,n) = (2,5) doesn't.
If m + n = p, then mn = (m+n) + 1
mn - m = n + 1
m(n - 1) = n + 1
m = (n + 1) / (n - 1)
At this point, I realized that n = 2 or n = 3. Otherwise, m will not be a positive integer.
So, (m,n) = (3,2) or (2,3)
Statement 1: Both m and n are prime numbers.
(m,n) = (2,3) satisfies the equation above.
(m,n) = (2,5) doesn't satisfy the equation.
Statement 2: p + 1 and m are both even.
(m,n) = (2,3) satisfies the equation above.
(m,n) = (2,5) doesn't.
Statement 1 & 2:
(m,n) = (2,3) satisfies the equation.
(m,n) = (2,5) doesn't.
- karthikpandian19
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OA is E
Regards,
Karthik
The source of the questions that i post from JUNE 2013 is from KNEWTON
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Karthik
The source of the questions that i post from JUNE 2013 is from KNEWTON
---If you find my post useful, click "Thank" ---
---Never stop until cracking GMAT---