First of all, if you just look at the angles, you can already tell that angle A is the right angle, in which you can then cross out A and C. Now you are down to B or D. This was tough but if you remember that the longer leg is square root of 3 times the shorter leg then you can tell that the included angle between the side showing 12 and the side showing 24 is the 60 degree angle. This leaves you to tell that the other angle is 30 degrees.
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