two cars, A & B, are 400 m apart. Car A travels due east at 30 m/s on a collision course with Car B, which travels due west at 20 m/s. How much time elapses before the two cars collide?

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Let the position function of car A is S(A)=30.t  where t is the time. So when t=0 the position of carA is 0.The position of B =S(B)=-20t+400When they collide they have the same positionso 30t=-20t+400solving for t =8sec