tricky DS question on num properties 700+ . Wat am i doing?

[san2009]

I am running into problem with statement 1 + statement 2 scenario:

OA is C

If a and b are positive integers what is the value of a+b?
1) a/b = 5/8
2) The greatest common divisor of a and b is 1

A>0, B>0 and both are integers
a + b= ?

1. this is just a ratio, won't get us a + b
the best it does for us is to rephrase the question in terms of either variable
insuff

2. thus, a and b have no common prime factors. but we know nothing abt value of a and b

1. + 2.

if a and b have no prime factors in common, CAN they can STILL have other factors in common?
I guess the answer to that is NO
because all numbers are composed of prime factors
and a and b don't have any prime factors in common, then that implies they have nothing in common
thus, a = 5 and b = 8
suff

Please correct my thinking...thanks

[selango]

a+b=?

stmt1,

a/b=5/8-->a/b=5x/8x

The ratio is given.There are many values possible for a and b.

Insuff

stmt1,

GCF(a,b)=1

There are many values possible for a and b.

Insuff

Combining 1 and 2,

a/b=5x/8x

GCF(a,b)=1

Whatever value of x,GCF(5x,8x)=x.Since GCF(5x,8x)=1 only possible values are 5 and 8[x=1]

a+b=13

Suff

Pick C

Hope this clarify..