Summarize the following:
Many rulers have two sets of numbers. You are looking for the metric set of numbers. This side of the ruler will be labeled cm{\displaystyle cm}, the abbreviation for centimeters. It might also be labeled mm{\displaystyle mm}, the abbreviation for millimeters.  Millimeters are the smaller metric units shown on the ruler by the shorter lines in between the centimeters.  The other side of the ruler is the standard side of the ruler. It will show inches, which is the standard U.S. unit. If you look closely at your ruler, you will see that each centimeter is divided into 10 parts by the shorter lines. A millimeter is one-tenth of a centimeter. Note that 1 mm = 0.1 cm. Look at the other edge of the object. Identify the nearest mark to that edge. This mark will tell you how long the object is.  If the edge falls on a longer centimeter line marked with a whole number, the object is that many centimeters long. For example, if the edge of an eraser ends at the line marked 7, the eraser is 7 cm long. If you are measuring to the nearest centimeter, look for the nearest line to the edge of the object that is marked with a number. This mark will tell you the nearest centimeter. For example, an eraser might fall less than halfway between the 7 and 8 cm marks. Since the eraser ends closer to the 7 cm mark, you would say that, to the nearest centimeter, the eraser is 7 centimeters long. Look at the last whole centimeter the edge of the object passes. Then, count the number of millimeters past the last whole centimeter that the object’s edge goes. Each millimeter is one-tenth of a centimeter. So, to find the length, combine the number of whole centimeters and tenths of a centimeter.For example, if the edge of an eraser ends 1 millimeter past 7 centimeters, the eraser is 7.1 centimeters long.
Locate the centimeters on the ruler. Understand the relationship between a millimeter and a centimeter. Find the length in whole centimeters. Find the length to the nearest tenth of a centimeter.