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Option 2 : 40 km/h
Calculation:
Let the speed of the first car be V1, the second car be V2 and the third car be V3.
Let the distance between A and B = X
⇒ X/ 2V1 = X/ V2
⇒ V2 = 2V1
Total distance of A to C = 100 + X
Distance covered by third car = 100 + X + ((100 + X)/ 2)
⇒ 100 + X/ 2 V1 = 3(100 + X)/ 2V3
⇒ 1/ V1 = 3/ V3
⇒ V3 = 3 × V1
⇒ V1 + V2 + V3 = 100
⇒ V1 + 2V1 + (3 × V1) = 120
⇒ 6V1 = 120
⇒ V1 = 20 and V3 = 60
⇒ V3 – V1 = 40 km/h
∴ The difference between the speed of the first car and the third car is 40 km/h
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