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Let s_{a}_{ }and s_{b }be the distance covered by car A and car B respectively.

v_{a}_{ }= s_{a}/t_{a}

S_{a }= v_{a}t_{a}

Similarly, s_{b} = v_{b}t_{b}

Since both cars are moving toward each other and are 100m apart, they would both pass each other when the can both cover a distance of 100m. That is,

s_{a} + s_{b }= 100

v_{a}t_{a} + v_{b}t_{b} = 100

Since both cars started at the same time,

t_{a} = t_{b} = t

v_{a}t + v_{b}t = 100

40t + 60t = 100

100t = 100

t = 100/100

t = 1 second

Time for both cars to be 100m apart = time taken to pass each other + time taken to cover another 100m

= 1 + 1

= 2 seconds

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