### Theory:

A number is a mathematical object which is used to count and measure.
Let's recall the classification of numbers that we learned in the previous grades.

Important!
• Every natural number is a whole number, integer and rational.
• Every whole number is an integer and rational.
• Every integer is a rational number.
Here come the pictorial forms of the classification of numbers:

By the definition of a subset of a set, the following result is obvious.

$\mathrm{ℕ}\subset W\subset \mathrm{ℤ}\phantom{\rule{0.147em}{0ex}}\subset \mathrm{ℚ}$