# CBSE Class 11-science Answered

Two cars A and B are moving with same speed 30 km/hr in same direction with car B 5 km behind the car A. Another car C moving in opposite direction crosses the two cars at an interval of 4 minutes. Find the speed of car C.

Let the car C is moving with velocity u km/hr. As the car B is 5 km behind the car A, therefore when the car C crosses the car A, it is 5 km from car B. Let us take left direction positive and right direction negative.

Therefore, velocity of car B = -5 km/hr

velocity of car C = + u km/hr

The relative velocity of car C w.r.t. car B

v = velocity of car C - velocity of car B

v = u - (-5) = (u+5) km/hr.

Now the car C travels 5 km distance with relative velocity (u+5) km/hr in 4 minutes.

Therefore, by using the relation, s = ut, we have

5 = (u+30) x (4/60)

or, 75 = (u+30)

or, u = 45 km/hr