Quick Answer: 21. Complete Solution: The numbers painted on each end of a runway tell a pilot the compass direction in which to land the plane. For example, consider a runway with ends that face north and south. The north end is labeled with the number 18. Placing a 0 after the 18 to make 180 indicates that a plane landing at this end would be heading south at 180°. In the challenge, the runway with the missing number is labeled 3 at its opposite end. The number 3 indicates a heading of 30°. Since each runway is a straight line, the difference between the headings for its two ends is always 180°. So, if one end requires a heading of 30°, the other corresponds to a heading of 30° + 180°, or 210°. The runway sign, therefore, should be 21.
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