Units & measurement

To convert units, multiply by a conversion factor that equals 1. A conversion factor is a fraction where the top and bottom represent the same measurement in different units, such as $\frac{1\text{ mile}}{1.609\text{ kilometers}}$. Choose the fraction that cancels the unit you have and leaves the unit you want. For example, to convert kilometers to miles, multiply by $\frac{1\text{ mile}}{1.609\text{ kilometers}}$ so the kilometers cancel. Place value tells you the value of a digit based on its position in a number. In $58,129,304$, the digit $5$ is in the ten-millions place, $0$ is in the hundreds place, $1$ is in the hundred-thousands place, $8$ is in the millions place, and $2$ is in the thousands place. To name a whole number, read each group of three digits followed by its group name, such as million or thousand, ignoring the ones group name. $3,975,284$ is three million, nine hundred seventy-five thousand, two hundred eighty-four. The symbol $\le$ means "is less than or equal to," so $3\le 20÷4$ reads "3 is less than or equal to 20 divided by 4."