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#### Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.  # x and y are prime numbers such that 9 < x < 100 and y is the reverse

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Math Expert V
Joined: 02 Sep 2009
Posts: 60550
x and y are prime numbers such that 9 < x < 100 and y is the reverse  [#permalink]

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Difficulty:   75% (hard)

Question Stats: 42% (03:01) correct 58% (02:32) wrong based on 31 sessions

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x and y are prime numbers such that 9 < x < 100 and y is the reverse of the digits of x, what is the value of xy?

(1) x - y = 18

(2) x + y = 44

Are You Up For the Challenge: 700 Level Questions

_________________
Senior Manager  P
Joined: 25 Jul 2018
Posts: 468
Re: x and y are prime numbers such that 9 < x < 100 and y is the reverse  [#permalink]

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x and y are prime numbers such that 9 < x < 100 and y is the reverse of the digits of x, what is the value of xy?

(Statement1): x —y = 18
x = 10a+ b
y= 10b+a

x —y = 10a+ b—10b—a=9a—9b=18
—> a —b= 2

31–13= 18
97 —79= 18
Insufficient

(Statement2): x+ y = 44
10a +b +10b+a= 11a +11b= 44
a+ b= 4
—> 3 + 1= 4
—> 1+3=4
31+13= 44
13+31=44
x*y= 31*13= 13*31
Sufficient

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GMAT Club Legend  V
Joined: 18 Aug 2017
Posts: 5701
Location: India
Concentration: Sustainability, Marketing
GPA: 4
WE: Marketing (Energy and Utilities)
Re: x and y are prime numbers such that 9 < x < 100 and y is the reverse  [#permalink]

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Bunuel wrote:
x and y are prime numbers such that 9 < x < 100 and y is the reverse of the digits of x, what is the value of xy?

(1) x - y = 18

(2) x + y = 44

Are You Up For the Challenge: 700 Level Questions

given x & y are prime no
#1
10x+y-10y-x=18
x-y=2
we can have (31,13) '; ( 35,53) ; ( 79,97)
insufficient
#2
x+y=44
x+y=4
13, 31
sufficient
IMO B
Manhattan Prep Instructor G
Joined: 04 Dec 2015
Posts: 870
GMAT 1: 790 Q51 V49 GRE 1: Q170 V170
Re: x and y are prime numbers such that 9 < x < 100 and y is the reverse  [#permalink]

### Show Tags

Bunuel wrote:
x and y are prime numbers such that 9 < x < 100 and y is the reverse of the digits of x, what is the value of xy?

(1) x - y = 18

(2) x + y = 44

Are You Up For the Challenge: 700 Level Questions

There are some serious fireworks going on in the question stem! Let's start there.

We know that x is between 9 and 100. That's a complicated way of saying that x is a 2-digit number. So, y will also be a 2-digit number. x and y are the reverse of each other, and x and y are both prime.

My rule of thumb is that if I think there are 10 or fewer total possibilities that match the constraints, I'll just write them all down before I start. I think that's probably the case here. How many prime numbers can there possibly be where the reverse is ALSO prime? Probably not too many, right?

Just to be safe, I'll reduce the possibilities a little bit more so I don't need to spend as much time looking. I don't need to look at any prime numbers that start with an even digit, since their opposite will be even, and therefore it won't be prime. I also don't need to look at any prime numbers that start with 5, since the opposite will be a multiple of 5, and won't be prime.

So I only need to think about prime numbers that start with 1, 3, 7, or 9. I can jot those down pretty fast:

11, 13, 17, 19
31, 37
71, 73, 79
97

Now, which ones are still primes when they're reversed? Here are the possibilities:

11, 11
13, 31
17, 71
37, 73
79, 97

Five possibilities. I was right, there are fewer than 10!

Each of these pairs will have a different product, and the question asks for the product. So, a statement will be sufficient if it lets us narrow things down to exactly one of these pairs. I'd rather add than subtract, so I'll start with statement 2.

Statement 2: x + y = 44. There's only one pair up there that sums to 44, and that's 13 and 31. (It doesn't matter which is x and which is y, since the product would be the same either way, and that's what we're trying to solve for.) So, this statement is sufficient. Eliminate A, C, and E.

Statement 1: x - y = 18. Yikes - I see two pairs that have a difference of 18. 13 and 31 works, but so does 79 and 97. So, I can't narrow it down to just one pair. Therefore, this statement is insufficient.

_________________ Chelsey Cooley | Manhattan Prep | Seattle and Online

My latest GMAT blog posts | Suggestions for blog articles are always welcome! Re: x and y are prime numbers such that 9 < x < 100 and y is the reverse   [#permalink] 07 Nov 2019, 12:41
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