Is this a rt triangle

Yes, if the sides are in the ratio 3:4:5 the triangle must be a pythagorean triplet right triangle. The sides will be some multiple of that ratio such as 6, 8 and 10 etc.

This can be easily proved by observing that if the sides are 3x, 4x and 5x => (3x)^2 + (4x)^2 = 9x^2 + 16x^2 = 25x^2 = (5x)^2... which is simply a verification of the pythagorean theorem - this is sufficient to figure out that the triangle is right-angled.

This is why statement 1 is sufficient.

Statement 2 is easier: The sum of the angles must be 180

=> 3x + 4x + 5x = 180 => 12x = 180 => x = 15

Thus the angles are 45, 60 and 75... which means the angle is NOT right-angled.

This is unusual for the GMAT that the two statements contradict eachother. However, this is still fair-game as the GMAC doesn't officially dismiss this possibility.

Pick D.