# Add & Subtract Units of Length

Learning how to add and subtract within units of measurement is a second grade, Common Core math skill: 2.MD.5. Below we show two videos that demonstrate this standard. Then, we provide a breakdown of the specific steps in the videos to help you teach your class.

Prior Learnings

Your students will have learned how to measure objects through non-standard units, like using paperclips to measure a pencil (1.MD.1). They should also be able to compare objects using terms like longer, shorter, longest, and shortest (1.MD.2).

Future Learnings

In the future, understanding how to measure objects with a ruler will help your students expand on and apply the concept elsewhere. Students will be able to make a line plot, measure objects, and place those measurements on the plot (2.MD.9). Your students will also be able to use rulers marked with halves and fourths of an inch (3.MD.3) and apply “linear measurement to measure perimeter and area” (3.MD.5- 8).

Common Core Standard: 2.MD.5 - Use addition and subtraction within 100 to solve word problems involving lengths that are given in the same units

Students who understand this principle can:

1. Understand that when adding or subtracting lengths, the units must be the same.
2. Represent measurement problems using drawings and equations.
3. Add or subtract within 100 to solve word problems involving length.
4. Write an equation for a measurement word problem, representing the unknown number with a symbol.

Video 1: Rules on Adding and Subtracting Units

This 1 minute video shows how to subtract units from a measurement and provides some rules about adding and subtracting with measurements.

The video begins by explaining that when measuring things, you sometimes want to add or subtract from the measurement. The video then gives a hypothetical scenario:

1. Using a ruler, you measure a stick.
2. The stick is 11 inches long.
3. What would happen if you cut 3 inches off the stick?
4. You can write an equation to model the scenario
5. 11 inches - 3 inches = 8 inches.

The video then explains that students must write inches next to each number, and the equation would not work if one of the units changed.

1. The word next to the number is called the “Unit”
2. The units must match.
3. An equation like “10 centimeters + 23 feet = ____ inches” cannot work.
4. You can only add or subtract numbers that have the same unit.
5. The answer will also have the same unit next to it.

Video 2: Add Measurements by Solving a Word Problem

The video briefly explains what it is about and then moves straight into the problem. The word problem is below:

“Jean and Peter are assembling a model train set. Jean puts down 10 feet of a red track. Peter then puts down 12 feet of a blue track. How long is the train track in all?”

The video explains that you can add units of measurement the same way you add numbers.

1. Jean puts down 10 ft of track.
2. Peter puts down 12 ft more of track.
3. To find the total length, add 10 and 12.
4. 10 + 12 = 22
5. Meaning that the train track is 22 ft long.

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