Question

in the figure, O is the center of a circle passing through A, B, C and D and angle ADC=120. Find the value of x?

Answer 1

its simple 
x+120=180
x=60

Answer 2

x=30
as 
cda +cba =180 opp angles in a cyclic quad
cba=60
acb=90     angles in a semicircle=90