First, we have to see what the least difference between any could be. We find the least distance to be possible, for example, when there is 64 and 60, because the difference is there 4 and otherwise, it would be a higher number.

I will list out the numbers for anyone who wants to try and prove me wrong (not that I'm saying you're a bad community, this is a great one) :

Sequence A: 2, 4, 8, 16, 32, 64, 128, 256, 512

Sequence B: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320

The answer is 4