Special Relativity - Space-Time Interval

In inertial frame S, two events occur at the same instant in time and 3c·minutes apart in space. In inertial frame S’, the same events occur at 5c·minutes apart. What is the time interval between the events in S’?

A. 0 min
B. 2 min
C. 4 min
D. 8 min
E. 16 min

(GR9677 #50)

Solution:

dS²  =  (dx)² − (cdt)²

dS²  =  dS'²

(3c)²  =  (5c)² − (ct)²

9c²  =  25c² − (ct)² 

t²  =  25 − 9 = 16 

t = 4

Answer: C