The number "0.4" is a decimal that represents four-tenths of a whole. In fraction form, it can be expressed as 2/5, meaning if you divide something into five equal parts, 0.4 corresponds to two of those parts. It is commonly used in mathematics, science, and everyday measurements. In terms of percentages, "0.4" is equivalent to 40%. This means that if you have a total of 100 units, 0.4 represents 40 units. Understanding decimals like 0.4 is essential for calculations in various fields, including finance, statistics, and cooking.