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Re: What is the two-digit number N? [#permalink]
03 Nov 2012, 09:07

3

This post received KUDOS

Expert's post

What is the two-digit number N?

Any two-digit integer can be represented as 10a+b (wher a and b are singel digit integers), for example 37=3*10+7, 88=8*10+8, etc.So, let's say N=10a+b.

(1) The difference between N and the number formed by reversing its digits is 9 --> (10a+b)-(10b+a)=9 --> a-b=1 --> N can be: 21, 32, ... Not sufficient.

(2) The number N is divisible by 9 --> in order a number to b divisible by 9, the sum of its digit must b divisible by 9. Thus we are given that a+b=9 (in this case N can be 18, 27, 36, 45, 54, 63, 72, 81, or 90) or a+b=18 (in this case N can only be 99). Notice that a+b cannot be a multiple of 9 more than 18, since a and b are single digit integers. Not sufficient.

(1)+(2) N cannot be 99 (a+b=18), since 99-99=0 not 9 as (1) states. So, we have that a-b=1 and a+b=9 --> a=5 and b=4, thus N=54. Sufficient.

Re: What is the two-digit number N? [#permalink]
04 Nov 2012, 11:32

Bunuel wrote:

What is the two-digit number N?

Any two-digit integer can be represented as 10a+b (wher a and b are singel digit integers), for example 37=3*10+7, 88=8*10+8, etc.So, let's say N=10a+b.

(1) The difference between N and the number formed by reversing its digits is 9 --> (10a+b)-(10b+a)=9 --> a-b=1 --> N can be: 21, 32, ... Not sufficient.

(2) The number N is divisible by 9 --> in order a number to b divisible by 9, the sum of its digit must b divisible by 9. Thus we are given that a+b=9 (in this case N can be 18, 27, 36, 45, 54, 63, 72, 81, or 90) or a+b=18 (in this case N can only be 99). Notice that a+b cannot be a multiple of 9 more than 18, since a and b are single digit integers. Not sufficient.

(1)+(2) N cannot be 99 (a+b=18), since 99-99=0 not 9 as (1) states. So, we have that a-b=1 and a+b=9 --> a=5 and b=4, thus N=54. Sufficient.

Answer: C.

Hope it's clear.

Even when we combine both the statements, N can be 54 or 45, right? So isn't the answer E?

Re: What is the two-digit number N? [#permalink]
04 Nov 2012, 13:50

Expert's post

Argon wrote:

Bunuel wrote:

What is the two-digit number N?

Any two-digit integer can be represented as 10a+b (wher a and b are singel digit integers), for example 37=3*10+7, 88=8*10+8, etc.So, let's say N=10a+b.

(1) The difference between N and the number formed by reversing its digits is 9 --> (10a+b)-(10b+a)=9 --> a-b=1 --> N can be: 21, 32, ... Not sufficient.

(2) The number N is divisible by 9 --> in order a number to b divisible by 9, the sum of its digit must b divisible by 9. Thus we are given that a+b=9 (in this case N can be 18, 27, 36, 45, 54, 63, 72, 81, or 90) or a+b=18 (in this case N can only be 99). Notice that a+b cannot be a multiple of 9 more than 18, since a and b are single digit integers. Not sufficient.

(1)+(2) N cannot be 99 (a+b=18), since 99-99=0 not 9 as (1) states. So, we have that a-b=1 and a+b=9 --> a=5 and b=4, thus N=54. Sufficient.

Answer: C.

Hope it's clear.

Even when we combine both the statements, N can be 54 or 45, right? So isn't the answer E?

N cannot be 45, because 45-54=-9 not 9 as stated in (1). _________________

Re: What is the two-digit number N? [#permalink]
01 Jun 2013, 08:39

Bunuel wrote:

What is the two-digit number N?

Any two-digit integer can be represented as 10a+b (wher a and b are singel digit integers), for example 37=3*10+7, 88=8*10+8, etc.So, let's say N=10a+b.

(1) The difference between N and the number formed by reversing its digits is 9 --> (10a+b)-(10b+a)=9 --> a-b=1 --> N can be: 21, 32, ... Not sufficient.

(2) The number N is divisible by 9 --> in order a number to b divisible by 9, the sum of its digit must b divisible by 9. Thus we are given that a+b=9 (in this case N can be 18, 27, 36, 45, 54, 63, 72, 81, or 90) or a+b=18 (in this case N can only be 99). Notice that a+b cannot be a multiple of 9 more than 18, since a and b are single digit integers. Not sufficient.

(1)+(2) N cannot be 99 (a+b=18), since 99-99=0 not 9 as (1) states. So, we have that a-b=1 and a+b=9 --> a=5 and b=4, thus N=54. Sufficient.

Answer: C.

Hope it's clear.

quick question: doesn't the difference between two numbers mean the absolute value? The difference between 54 and 45 is 9, does it the same as the difference between 45 and 54?

Re: What is the two-digit number N? [#permalink]
11 Aug 2013, 06:37

kevinfa wrote:

Bunuel wrote:

What is the two-digit number N?

Any two-digit integer can be represented as 10a+b (wher a and b are singel digit integers), for example 37=3*10+7, 88=8*10+8, etc.So, let's say N=10a+b.

(1) The difference between N and the number formed by reversing its digits is 9 --> (10a+b)-(10b+a)=9 --> a-b=1 --> N can be: 21, 32, ... Not sufficient.

(2) The number N is divisible by 9 --> in order a number to b divisible by 9, the sum of its digit must b divisible by 9. Thus we are given that a+b=9 (in this case N can be 18, 27, 36, 45, 54, 63, 72, 81, or 90) or a+b=18 (in this case N can only be 99). Notice that a+b cannot be a multiple of 9 more than 18, since a and b are single digit integers. Not sufficient.

(1)+(2) N cannot be 99 (a+b=18), since 99-99=0 not 9 as (1) states. So, we have that a-b=1 and a+b=9 --> a=5 and b=4, thus N=54. Sufficient.

Answer: C.

Hope it's clear.

quick question: doesn't the difference between two numbers mean the absolute value? The difference between 54 and 45 is 9, does it the same as the difference between 45 and 54?

Agreed. In fact, I also chose E, because of this reason. Difference between 45 and 54 should always be 9, irrespective of the fact which comes earlier.

Re: What is the two-digit number N? [#permalink]
14 Oct 2014, 20:29

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