Third Grade Mathematics
Unit 2
Addition, Subtraction, and Measurement

Printable Parent Letter

During Unit 2, your children will add and subtract within 1000 by applying their understanding of models for addition and subtraction.  They will develop, discuss, and use efficient, accurate, and generalizable methods to compute the sums and differences of whole numbers in base ten notations, using their understanding of place value and the properties of the operations (they will need not use formal terms for these properties).  Your children will work to develop written methods for recording sums and differences. They will be introduced to the concept of rounding, which provides them with another strategy to judge the reasonableness of their answers in addition and subtraction situations.  Perimeter provides a context in which students can practice both rounding and addition and subtraction (e.g. estimating the perimeter of a polygon).  They will also develop a conceptual understanding of measuring mass, liquid volume and intervals of time. Measurement word problems will be used as a context for the development of fluency in addition and subtraction.

Students need to:

  • Add and subtract within 1000 using strategies and algorithms based on the following: place value, properties of operations and the relationship between addition and subtraction
  • Use place value understanding to round whole numbers to the nearest 10 or 100
  • Solve two-step word problems.  Represent these problems using equations with a letter standing for the unknown quantity. Assess the reasonableness of answers using mental computation and estimation strategies including rounding.
  • Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length.
  • Tell and write time to the nearest minute and measure time intervals in minutes.  Solve word problems involving addition and subtraction of time intervals in minutes, e.g., by representing the problem on a number line diagram.
  • Measure and estimate liquid volumes and masses of objects using standard units of grams (g), kilograms (kg), and liters (l). 
  • Add or subtract to solve one-step word problems involving masses or volumes that are given in the same units, e.g., by using drawings (such as a beaker with measurement scale) to represent the problem.

Carroll County Public Schools Video Support

Addition with Open Number Lines

Addition with Regrouping

Subtraction with Open Number Lines

Subtraction with Regrouping

Subtraction Across Zeros

Stacking Addition
Stacking Method With Addition

Stacking Subtraction
Stacking Method With Subtraction


Ways Parents Can Help

  • Help your child use addition or subtraction to solve real world problems (e.g. adding a bill, calculating change from a purchase…)
  • Practice reading an analog clock.
  • Help your child determine an end time given the start time and the duration of the event (e.g. you put something in the oven at 5:15 p.m. and it needs 32 minutes to cook, what time should you take it out of the oven).
  • Look at real world examples showing liquid volumes and masses (e.g. packaged food such as a cereal box). Play a game to see how close your child can get to estimating these measurements.

Some Support Sites



Key Vocabulary to Know

Addend - the numbers in a addition problem to be added together

Addition - the action of joining numbers together

Associative Property - regrouping addends or factors will not impact to total.  

Commutative Property - changing the order of addends or factors does not change the total. 

Difference - the answer to a subtraction problem

Digit - a numeric symbol that is used to represent numbers

Estimate - an approximate total

Equal - having the same value or being the same size

Identity Property - the value of a factor does not change when multiplied by 1, nor does an addend when zero is added.

Inverse operation - the opposite

Minuend - the number in which another number is subtracted from

More - a greater value or larger size

Multiples of 10 and 100 - when a number is multiplied by 10 or 100

Number line - a line in which the numbers mark intervals

Operation - addition, subtraction, multiplication, and division

Place value - the value of a digit based on it's position in a number

Standard algorithm - the traditional method of computing numbers

Subtraction - the action of taking the value of one number away from another

Subtrahend -  the number to be subracted in a problem

Sum - the answer to an addition problem

Capacity - the amount of liquid an object and hold

Gram - metric unit of mass (weight)

Elapsed time - the amount of time that has passed.

Height - the quality of being tall

Kilogram - a metric unit of mass (weight) 1,ooo grams

Liter - metric unit of capacity (volume)

Mass - weight

Measure - a standard unit used to express the size, amount, or degree of something.

Milliliter - metric unit of capacity (volume) 1/1000 of a liter

Minutes - measurement of time 60 seconds

Perimeter - the distance around the outside of an object

Scale - the intervals in which a bar graph is labeled or the value of the image on a pictograph.

Standard unit - an established common unit (ie: foot, inch, cm)