With (2x+3x+7x)=360 x=12, so the angles are 60, 90, and 210. I don't see how the LCD of 2,3,7 or 60,90,210 is 15. Wouldn't it be 42 and 630, respectively? If you're talking about least common multiple (LCM) of the actual angles it wouldn't be 15 either, it would be 3 (although 15 is a multiple). Where did the 15 come from?