Noelle walks from point A to point B at an average speed of 6 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 7 kilometers per hour?
Choose the correct choice from the following choices
A) 6.75	B) 7	C) 7.25	D) 7.5	E) 8.4

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 7 = 2 * 6 * x / 6 + x x = 8.4 e is the answer"
Answer: E