Define a function that when passed three floating-point values, the first representing distance between two locations, the second representing the average speed of one-way trip, and the third representing the required average speed of the entire return-trip, returns the speed at which you must return to achieve the expected overall average.

For example, if A and B are 60 kms apart, you travel from A to B @ 30kmph, and the overall average speed should be 40kmph, then you must return from B to A @60kmph. We will let you work out the math.