The number "3.99" is a decimal number that represents a value slightly less than four. In everyday contexts, it is often used to indicate prices, such as in retail settings where an item may cost $3.99. This pricing strategy is common because it can make products appear more affordable to consumers. In mathematics, "3.99" can also be expressed as a fraction, specifically 399/100. It is a rational number, meaning it can be represented as the quotient of two integers. Additionally, "3.99" can be rounded to "4" when considering whole numbers, which is useful in various calculations and estimations.