47. Car A moves at a constant speed of $8\,\dfrac{\text{m}}{\text{s}}$ along a straight path, and car B moves behind it at a constant speed of $20\,\dfrac{\text{m}}{\text{s}}$ in the same direction. When the distance between them decreases to 46 meters, car A begins to decelerate with a constant acceleration of $2\,\dfrac{\text{m}}{\text{s}^2}$ and simultaneously car B also decelerates with a constant acceleration of $4\,\dfrac{\text{m}}{\text{s}^2}$. What is the speed of car B at the moment it reaches car A, in meters per second? (1) $2$ (2) $8$ (3) $4$ (4) $6$
\textbf{47.} Car A moves at a constant speed of $8\,\dfrac{\text{m}}{\text{s}}$ along a straight path, and car B moves behind it at a constant speed of $20\,\dfrac{\text{m}}{\text{s}}$ in the same direction. When the distance between them decreases to 46 meters, car A begins to decelerate with a constant acceleration of $2\,\dfrac{\text{m}}{\text{s}^2}$ and simultaneously car B also decelerates with a constant acceleration of $4\,\dfrac{\text{m}}{\text{s}^2}$. What is the speed of car B at the moment it reaches car A, in meters per second?
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(1) $2$ \hspace{2cm} (2) $8$ \hspace{2cm} (3) $4$ \hspace{2cm} (4) $6$
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