v/u = 2
I assume that u is the speed in the first half and v the speed in the last half. Let d be the distance from A to B. Then the distance from A to mid of AB is d/2 and distance from mid of AB to B is also d/2. And let the time to go from mid of AB to B be t. Then the time to go from A to mid of AB is 2*t.
So, when it goes that distance in half the time, it is going twice as fast!
I hope this helps,