Well, I dont want the solution but a few ideas to start solving this.. Two unstable particles move in the reference frame M with a velocity v=0.992c. The distance between them in this reference frame is l=110 m. At a certain moment both particles decay simultaneously in this frame. What time interval between the moments of decay of the 2 particles will be observed in the frame ? Which particle decays later in this frame? Please tell me a bit on the concept of particle decay in a relativistic case like this..