It is always isosceles because it can be proved as follows:
The perpendicular bisector dissects the triangle into two, and it is the common side. Then each side of the bisector is 90 degrees, and the bisected to two equal sides, so the two dissected triangles are congruent, hence the original triangle is isosceles.
1 is the number which is the answer
Their error is that they divided 12 by 30, instead of 30 by 12 in order to get the price per pair
the correct unit price is $2.50 per pair
2.5 * 12 = 30
