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设ç²ä¹ä¸¤å°å ¨é¿ä¸ºS
åæå»æ¶æ¶é´ä¸ºS/40
ååæ¶æ¶é´ä¸ºS/50
2S÷(S/50+S/40)
=2S÷(4S/200+5S/200)
=2S÷9S/200
=400/9(km/å)
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