Let the base angle be x. If the obtuse angle is two and a half times the base angle, it will be 2.5x. In a triangle, all three angles will add up to 180. The third angle, the other base angle will be equal to x. So the equation will be:
x + x + 2.5x = 180 2x + 2.5x = 180 4.5x = 180 x = 40
