A, B and C begin together to move around in a circular stadium and they complete their revolution in 42 sec, 63 sec, and  84 sec respectively.  After how much time they will come together at the starting point?

152 sec 252 sec 352 sec 192 sec

252 sec

LCM of 42, 63, 84 = 252  ∴ They will come together at the starting point after 252 seconds