Hi, I am just trying to wrap my head around a quiz question here. I ca figure it out through deduction but I would really like to understand it. The question gives a right triangle ADB with AD as the leg, AB as the hypotenuse and angle ADB as the right angle. It is asking for the measure of angle DAB knowing only that line segment AD is 2/3 as long as line segment AB.

What rule or theorem applies and why is that? How do go about this? Thanks for any help, appreciated.