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TWO MINUTES. IT TAKES THE FRONT OF THE TRAIN ONE MINUTE AND THE REST OF THE TRAIN WILL TAKE TWO MINUTES TO CLEAR THE TUNNEL IF THE GIRLS HAD BEEN ON A STANDING TRAIN, THE FIRST GIRL'S CALCULATIONS WOULD HAVE BEEN CORRECT, BUT THEIR TRAIN WAS MOVING. IT TOOK 5 MINUTES TO MEET A SECOND TRAIN, BUT THEN IT TOOK THE SECOND TRAIN 5 MORE MINUTES TO REACH WHERE THE GIRLS MET THE FIRST TRAIN. SO THE TIME BETWEEN TRAINS IS 10 MINUTES, NOT 5, AND ONLY 6 TRAINS PER HOUR ARRIVE IN THE CITY FOR THE TRAIN TO PASS COMPLETELY THROUGH THE TUNNEL, IT MUST TRAVEL 2 MILES. AFTER 1 MILE TRAVEL, THE TRAIN WOULD BE COMPLETELY IN THE TUNNEL, AND AFTER ANOTHER MILE IT WOULD BE COMPLETELY OUT AND SINCE THE TRAIN IS TRAVELING AT 1 MILE A MINUTE, IT WILL TAKE 2 MINUTES TO PASS THROUGH THE TUNNEL 25 KILOMETERS. THE PLANES ARE TRAVELING AT A COMBINED SPEED OF 25 KILOMETERS PER MINUTE. SO THEY WILL BE 25 KILOMETERS APART ONE MINUTE BEFORE THEY MEET. THE INITIAL DISTANCE BETWEEN THE PLANES IS IRRELEVANT