PQ = ST QR = RS The goal is to find x, the measure of one of the angles formed by the intersection of ST and PT. Now angle QRS is labeled 80°. You also know PQ and ST have the same length and QR and RS have the same length. If you add PQ and QR, you get PR. If you add ST and RS, you get RT. If you add equals to equals, you get equals, so PQ + QR must be the same as ST + RS, which means that PR and RT are the same. Thus, you have isosceles triangle PRT, and you’re given one angle that has measure 80 and a second angle that has measure x. The angle measuring x is opposite equal side PR. That means the other angle must have the same measure. The sum of the interior angles in a triangle always equals 180°. Thus, x + x + 80 must equal 180, 2x = 100, and x = 50. The answer is (B).