The perimeter of an equilateral triangle is 60. If one of the sides of the equilateral triangle is the side of an isosceles triangle of perimeter 55, then how long is the base of isosceles triangle?  Pick one:
A) 5 B) 10 C) 15 D) 20 E) 25

Rationale: "the base of the isosceles triangle is 55 - 20 - 20 = 15 units the answer is c ."
Answer: C