value of A+B? and GCD=1

[blover]

http://gmatclub.com/forum/download/file.php?id=7312

[e.cartman]

Imo C.
Stmt1: 8a=5b => a=5k, b=8k will work. insuff
Stmt2: a and b are mutually prime. By itself insuff.
Together a=5, b=8.

[bose]

yep c it is
a=5, b=8
from 1 we do not know a and b as a can be 5x and b=8x.
from 2 we do not know anything abt a and b
from 1 and 2 we know a=5 b=8 as x=1

[ankitharish]

got to be c.
Since a and b are positive remember v can cross multiply.so 8a = 5b.
a can be 5,10,15 and respective value of b can be 8,16,24.
from stmnt 2 gcd =1 which is only possible if a = 5 n b=8.

[genius_in_the_gene]

1) 8a=5b.
Means a =5 when b=8, a=10 when b=16 etc. Not sufficient.

2) GCD of a and b is 1. Means a and b are coprime.

So a and b cant have any common factors except 1.

Combining 1 and 2, a=5, b=8 and vice versa. Hence C