Apparently it's a very easy question. But somehow I can't figure out where I am going wrong. Any help will be greatly appreciated.

Two observers in the S frame, A and B, are separated by a distance of 60 m. Let S' move at a speed 3c/5 relative to S, the origins of the two systems, O' and O, being coincident at t' = t = 90/c s. The S' frame has two observers, one at A' and one at a point B' such that, according to clocks in the S frame, A' is opposite to A at the same time that B' is opposite B. What is the reading on the clock of B' when B' is opposite B?

Question

Apparently it's a very easy question. But somehow I can't figure out where I am going wrong. Any help will be greatly appreciated.

Two observers in the S frame, A and B, are separated by a distance of 60 m. Let S' move at a speed 3c/5 relative to S, the origins of the two systems, O' and O, being coincident at t' = t = 90/c s. The S' frame has two observers, one at A' and one at a point B' such that, according to clocks in the S frame, A' is opposite to A at the same time that B' is opposite B. What is the reading on the clock of B' when B' is opposite B?

anonymous · Answer

what does "clock" refer to here?

anonymous · Answer

The reading of the clock in a reference frame at some point in space means the time coordinate measured  by an observer who is standing at that point at rest relative to that reference frame.