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If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Given: F+1=2S. Question: S=?

(1) The digits MN making up the father's age are reversed in the son's age i.e NM --> F=10M+N and S=10N+M (notice that M and N must be digits: 0, 1, 2, ..., 9) --> (10M+N)+1=2*(10N+M) --> 8M=19N-1. Only one set of digits satisfy this equation: M=7 and N=3 (by trial and error: 19N-1 is even and also a multiple of 8 --> N must odd --> so try N=1 and N=3 to see whether 19N-1 is a multiple of 8. For N>=5, M>10 so not a solution for M as it must be a single digit integer) --> S=10N+M=37 and F=73. Sufficient.

(2) Father is at least 25 years older than son. Clearly insufficient: S=25 and N=13 or S=27 and N=14 .

If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Given: F+1=2S. Question: S=?

(1) The digits MN making up the father's age are reversed in the son's age i.e NM --> F=10M+N and S=10N+M (notice that M and N must be digits: 0, 1, 2, ..., 9) --> (10M+N)+1=2*(10N+M) --> 8M=19N-1. Only one set of digits satisfy this equation: M=7 and N=3 (by trial and error: 19N-1 is even and also a multiple of 8 --> N must odd --> so try N=1 and N=3 to see whether 19N-1 is a multiple of 8. For N>=5, M>10 so not a solution for M as it must be a single digit integer) --> S=10N+M=37 and F=73. Sufficient.

(2) Father is at least 25 years older than son. Clearly insufficient: S=25 and N=13 or S=27 and N=14 .

Answer: A.

Hope it's clear.

Bunnel: will the equation here not be F-1 =2S ( Father's age is 1 less than twice the son's age)

If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Gimme Kudos if you like the question

Look at the big picture. You can solve this question quickly using intuition.

Father's age is almost as much as twice of son's age. (just 1 less) Son's age = NM, Father's age = MN

Say, son's age is 12 then father's is 21 (3 less than twice) Say, son's age is 13 then father's is 31 (much more than twice) I hope you understand that you can give up on the 10s series here because the gap will keep widening (diff b/w 14 and 41 will be even more)

Say, son's age is 24 then father's is 42 (much less than twice) Say, son's age is 25, then father's is 52 (more than twice so give up on the 20s series too)

Say son's age is 36, then father's is 63 (much less than twice) Say son's age is 37, then father's is 73 (1 less than twice. Got it!)

It's easy to say that there will be no other such combination because 49, 94 doesn't work. Nothing in 50s, 60s and so on will work because we need to stick to 2 digit numbers.

Hence, only statement 1 is sufficient.
_________________

If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Given: F+1=2S. Question: S=?

(1) The digits MN making up the father's age are reversed in the son's age i.e NM --> F=10M+N and S=10N+M (notice that M and N must be digits: 0, 1, 2, ..., 9) --> (10M+N)+1=2*(10N+M) --> 8M=19N-1. Only one set of digits satisfy this equation: M=7 and N=3 (by trial and error: 19N-1 is even and also a multiple of 8 --> N must odd --> so try N=1 and N=3 to see whether 19N-1 is a multiple of 8. For N>=5, M>10 so not a solution for M as it must be a single digit integer) --> S=10N+M=37 and F=73. Sufficient.

(2) Father is at least 25 years older than son. Clearly insufficient: S=25 and N=13 or S=27 and N=14 .

Answer: A.

Hope it's clear.

Bunnel: will the equation here not be F-1 =2S ( Father's age is 1 less than twice the son's age)

It's all about word translation: father's age is 1 less than twice the son's age, so F is less than 2S by 1 --> so we need to add 1 to F to get 2S --> F+1=2S.

Re: If Father's age is 1 less than twice the son's age, what is [#permalink]

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02 Oct 2014, 02:10

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Re: If Father's age is 1 less than twice the son's age, what is [#permalink]

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11 Jan 2016, 01:51

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If Father's age is 1 less than twice the son's age, what is [#permalink]

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23 Dec 2016, 15:08

Bunuel wrote:

docabuzar wrote:

If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Given: F+1=2S. Question: S=?

(1) The digits MN making up the father's age are reversed in the son's age i.e NM --> F=10M+N and S=10N+M (notice that M and N must be digits: 0, 1, 2, ..., 9) --> (10M+N)+1=2*(10N+M) --> 8M=19N-1. Only one set of digits satisfy this equation: M=7 and N=3 (by trial and error: 19N-1 is even and also a multiple of 8 --> N must odd --> so try N=1 and N=3 to see whether 19N-1 is a multiple of 8. For N>=5, M>10 so not a solution for M as it must be a single digit integer) --> S=10N+M=37 and F=73. Sufficient.

(2) Father is at least 25 years older than son. Clearly insufficient: S=25 and N=13 or S=27 and N=14 .

Answer: A.

Hope it's clear.

Hello can you elaborate if it is possible? how did we come up with F=10M+N and S=10N+M , Basically i understood how u solved it but i cannot comprehend the way we approach it initially

If Father's age is 1 less than twice the son's age, what is the son's age? A. The digits MN making up the father's age are reversed in the son's age i.e NM. B. Father is at least 25 years older than son

I m not sure of the answer but i think its "A"

Given: F+1=2S. Question: S=?

(1) The digits MN making up the father's age are reversed in the son's age i.e NM --> F=10M+N and S=10N+M (notice that M and N must be digits: 0, 1, 2, ..., 9) --> (10M+N)+1=2*(10N+M) --> 8M=19N-1. Only one set of digits satisfy this equation: M=7 and N=3 (by trial and error: 19N-1 is even and also a multiple of 8 --> N must odd --> so try N=1 and N=3 to see whether 19N-1 is a multiple of 8. For N>=5, M>10 so not a solution for M as it must be a single digit integer) --> S=10N+M=37 and F=73. Sufficient.

(2) Father is at least 25 years older than son. Clearly insufficient: S=25 and N=13 or S=27 and N=14 .

Answer: A.

Hope it's clear.

Hello can you elaborate if it is possible? how did we come up with F=10M+N and S=10N+M , Basically i understood how u solved it but i cannot comprehend the way we approach it initially

Thank you in advance

This is a way of writing a two-digit number. Any two-digit integer can be represented as 10a+b (where a and b are single digit integers), for example 37=3*10+7, 88=8*10+8, etc.
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