Answer: 80
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Step-by-step explanation:
There are probably many approaches here, but one way to do this is to extend segment DO to form a line. Check out the diagram below. This line is in red.
The goal is to have this line extend to intersect with line AB. Let's call this intersection point E.
In that diagram, I've marked the variable y as angle EOB. Notice that angles BEO and CDO are congruent alternate interior angles (since lines AB and CD are parallel). So that's where that extra 30 degree angle comes from.
Focus on triangle EOB. The three angles of any triangle always add to 180
E+O+B = 180
30+y+50 = 180
y+80 = 180
y = 180-80
y = 100
The last step is to solve the equation below for x
x+y = 180
x+100 = 180
x = 180-100
x = 80
This works because angles x and y are supplementary. They form a straight angle.