If a and b are positive itegers, what is the value of a + b?

HI All,

This DS question involves a handful of simple math "rules" and can be solved by TESTing VALUES.

We're told that A and B are positive integers. We're asked for the value of A+B

Fact 1: A/B = 5/8

This gives us the ratio of the two integers, but not the integers themselves. It also means that A is a multiple of 5 and B is the equivalent multiple of 8.

IF....
A = 5
B = 8
Then the answer to the question is 13

IF....
A = 10
B = 16
Then the answer to the question is 26
Fact 1 is INSUFFICIENT

Fact 2: The greatest common divisor of A and B is 1

IF....
A = 1
B = 2
Then the answer to the question is 3

A = 5
B = 8
Then the answer to the question is 13
Fact 2 is INSUFFICIENT

Combined, we know
A/B = 5/8; A is a multiple of 5 and B is the equivalent multiple of 8
The greatest common factor of A and B is 1

IF....
A = 5
B = 8
Then the answer to the question is 13

Since A and B are multiples of 5 and 8, respectively, we can stop working. If we take ANY multiple of the two (then those resulting numbers will have a GCF that is GREATER than 1

For Example....
A = 10
B = 16
These numbers have a CGF of 2. This does not match the "restriction" from Fact 2, so it's NOT a useable TEST. None of the possibilities beyond 5 and 8 will "fit" these restrictions, so the answer to the question MUST be 5+8 = 13
Combined, SUFFICIENT.

Final Answer:

GMAT assassins aren't born, they're made,
Rich

So was the answer C or E? My reasoning for C is:

Rearranging the first statement gives us A = 5B/8.

Given both A and B are integers, and that 5 is not divisible by 8, then B must be a multiple of 8.

Also, since B=0 would not be a positive integer value, then B must equal 8 (and so A=5), or 16 (which is 8*2, meaning A = 5*2 = 10), or 24 (which is 8*3, A= 5*3 = 15), or 32 (a = 5*4 = 20) etc.

Note that in every case A and B have the multiple of 8 in common (e.g. if B = 16, which is 8*2, then A = 5*[8*2]/8, which means both A and B will have 2 as a factor). Since statement 2 tells us that they only have 1 as a common factor, B must equal 8 and A must equal 5 (5*[8*1]/8). Answer = C.

I'd appreciate further comments and explanation on this question.

Hello ElCorazon

1) From this statement we can infer that 8a = 5b
so more than two variants are possible:
a =5 and b = 8
or a = 50 and b = 80 and so on
Insufficient

2) GCF (a, b) = 1
Insufficient because we know only that a and b is primes or both or one of them equal to 1

1+2) a and b can only be 5 and 8 because all other variant will share some common factor other than 1
for example GCF(50, 80) = 10

Sufficient
Answer is C

(1) Not sufficient as a/b=5/8 is a ratio and exact value of a and b cannot be concluded.

(2) The greatest common divisor of a and b is 1, means that there is no common factor of a and b.
Not sufficient.

(1) & (2) When taken together, a and b should not have any common factor, therefore a and b can be 5 and 8 respectively, as the greatest common factor between them is 1.

Therefore C

For my own learning sake..


If a and b are positive integers, what is the value of a + b?


(1) a/b=5/8

A/B can be 10/16, 20/38, etc. Not Sufficient

(2) The greatest common divisor of a and b is 1.

A and B have to be numbers that do not share a prime factor.
But there are many ways to come to that conclusion. Not Sufficient

(1 & 2)

5 and 8 are the only pair in the given ratio that have the GCF of 1.
Therefore a = 5 and b = 8 --> a + b = 13 Sufficient!

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