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.