Noelle walks from point A to point B at an average speed of 3 kilometers per hour. At what speed, in kilometers per hour, must Noelle walk from point B to point A so that her average speed for the entire trip is 5 kilometers per hour? Choose the correct choice from the following answers:
A) 15  B) 12.5  C) 10  D) 8  E) 7.75

Rationale: let ' s suppose that speed while returning was xkm / h since the distance is same , we can apply the formula of avg speed avg speed = 2 s 1 s 2 / s 1 + s 2 5 = 2 * 3 * x / 3 + x x = 15 a is the answer
Answer: A