This is a question from the UNSW 2015 math competition for secondary 1. Only 4% of Singapore students who took part managed to get the answer correct. Can you?

ACD = 180 deg (Sum of a triangle). From the diagram you know ACD, DAC n ADC, hence (180-2x)+(180-2x)+(180-x)=180 since ACD=DAC (isoceles triangle ACD) n ADC is angles on straight line with ADB. ADB=x (isoceles triangle ADB). Answer is 72.

Got the same answer as Rose. 72. haha.. redrew the three triangles to help see them better.