Spaceship A, moving away from the earth at velocity \(3 / 5 c\), is sending messages to spaceship B, which left the earth earlier at speed \(4 / 5 c\) in the same direction. The messages sent by A are contained in pulses sent by a laser on A, with the pulses separated by 100 femtoseconds in A's frame of reference. As B receives the pulses, what is the pulse separation according to the crew on B?