 Comparing the coordinates in each frame, we see that in Alice's frame her tachyon signal moves forwards in time (she sent it at an earlier time than Bob received it), and between being sent and received we have (difference in position)/(difference in time) = 360/150 = 2.4"c".In Bob's frame, Alice's signal moves back in time (he received it at "t′" = 270, but it was sent at "t′" = 500), and it has a (difference in position)/(difference in time) of 400/230, about 1.739"c".The fact that the two frames disagree about the order of the events of the signal being sent and received is an example of the relativity of simultaneity, a feature of relativity which has no analogue in classical physics, and which is key to understanding why in relativity FTL communication must necessarily lead to causality violation.