Two cars P and Q move with velocities →vP and →vQ towards north and east respectively.
Velocity of car P w.r.t. Q (i.e., velocity of car P for an observer on cars Q) i.e., →vPQ will be in which direction?
North - west
→vPQ=→vP−→vQ=→vP+(−→vQ)
∴vPQ will be in the northwest direction.