Triangle ABC is isosceles. What is the measure of angle C? A. 25°
B. 30°
C. 60°
D. 75°

The answer is D, 75°.

All the interior angles of a triangle add up to 180°. With this information, we can set up this equation:

(x + 5) + (3x) + (3x) = 180

Combine like terms.

(x + 5) + (3x) + (3x) = 180

7x + 5 = 180

-5 -5

7x = 175

/7 /7

x = 25

Now, since this is an isosceles triangle, we can easily substitute the value of x into 3x. 3 • 25 = 75 which is 75°.