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.