The three angles of a triangle measure x°, 2x°, and (3x-30)°. What is the value of x?

The angles of a triangle sum to $180°$, so $x + 2x + (3x - 30) = 180$. Combine like terms: $6x - 30 = 180$. Add 30: $6x = 210$. Divide by 6: $x = 35$, choice D. Check: the angles are $35°$, $70°$, and $75°$, which add up to $180°$. Choice A (30) gives angles $30°$, $60°$, and $60°$, summing to $150°$. Choice B (36) gives $36 + 72 + 78 = 186$. Choice C (25) gives angles $25°$, $50°$, and $45°$, which sum to only $120°$. Always verify by summing the three angles.