Real numbers are the set of all possible real world quantities. Another way of thinking
about real numbers is that they are the points on an infinitely long line. Every possible
value, both positive and negative, appears somewhere on the line. The symbol ℝ is used
for this set. ℝ contains all natural numbers (ℕ), integers (ℤ), rational numbers (ℚ) and
Real numbers are very useful for measuring. For example, if three people ran a race, their
times (in seconds) may be 15, 17.3 and 18.3454. These are all real numbers.