Mental Subtraction - Grade 3
In these lessons, we will learn how to describe and apply mental mathematics strategies for subtracting two 2-digit numerals, such as:
- subtracting and compensating
- use addition to subtract
- subtraction using doubles
Subtracting and Compensating
In this method, we take the subtrahend (second number) to the nearest multiple of ten, subtract and then compensate to get the answer.
Example:
69 − 28 =
28 is close to 30
69 − 30 = 39
39 + 2 more
So, 39 + 2 = 41
How to subtract in your head using a technique called compensation.
Use Addition to Subtract
In this method, we think in terms of addition and use addition to subtract.
Example:
To determine the difference between 62 and 45,
Think:
5 more than 45 will get me to 50, 10 more is 60
I`ve added 15 so far and 2 more is 62, so my difference is 17.
Think Addition, Story problems and subtraction.
Subtraction using Doubles
Use a doubles fact you know, to help find the difference.
Example:
62 – 30=
30 + 30 = 60
60 – 30 = 30
32 is 2 more than 30
So 62 – 30 = 32
Using Doubles and Building on Doubles is a mental math strategy that introduces students to the concept that the addition doubles and near doubles can be used for subtraction as well.
This video outlines the basic ideas behind using doubles and building on doubles.
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