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Fractions
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Hong Kong
Stage 2

Checking Reasonableness of Fraction Sums

Lesson
Practice
Lesson

You may have learned about using benchmarks to estimate the value of fractions.

Now watch this video to learn how to estimate when we add and subtract fractions.

When plotting fractions on a number line we use:

  • the denominator to divide the whole numbers into equal parts
  • the numerator to select the number of parts.

Knowing approximately how close they are to other known fractions is important for understanding the value of fractions.

Remember!

The denominator tells us how many parts make the whole.

The numerator tells us how many parts in the fraction.

Knowing how the number line is divided up helps us to understand the value of fractions.

Examples

Question 1

Rosey is working on a problem and has to calculate $\frac{1}{22}+\frac{15}{16}$122​+1516​. She says the answer is about $1$1. Let’s see if her estimate is reasonable.

  1. Let’s look at $\frac{1}{22}$122​. Which value below is the best estimate for $\frac{1}{22}$122​?

    $2$2

    A

    $\frac{1}{2}$12​

    B

    $0$0

    C

    $1$1

    D

  2. Now let’s look at $\frac{15}{16}$1516​. Which value below is the best estimate for $\frac{15}{16}$1516​?

    $\frac{1}{2}$12​

    A

    $1$1

    B

    $2$2

    C

    $0$0

    D

  3. Rewrite the fraction sum using the rounded values. Do not add the values together.

  4. Calculate the sum $0+1$0+1.

  5. Is Rosey's estimate reasonable?

    Yes

    A

    No

    B

Question 2

Dave is working on a problem and has to calculate $\frac{2}{26}+\frac{12}{25}$226​+1225​. He says the answer is about $\frac{1}{2}$12​. Let’s see if his estimate is reasonable.

  1. Let’s look at $\frac{2}{26}$226​. Which value below is the best estimate of $\frac{2}{26}$226​?

    $\frac{1}{2}$12​

    A

    $0$0

    B

    $2$2

    C

    $1$1

    D

  2. Now let’s look at $\frac{12}{25}$1225​. Which value below is a reasonable estimate for $\frac{12}{25}$1225​?

    $0$0

    A

    $\frac{1}{2}$12​

    B

    $1$1

    C

    $2$2

    D

  3. Rewrite the fraction sum using the rounded values. Do not add the values together.

  4. Calculate the fraction sum $0+\frac{1}{2}$0+12​.

  5. Is Dave's estimate reasonable?

    No

    A

    Yes

    B

Question 3

Bianca is working on a problem and has to calculate $\frac{11}{12}-\frac{16}{33}$1112​−1633​. She says the answer is about $\frac{1}{2}$12​. Let’s see if her estimate is reasonable.

  1. Let’s look at $\frac{11}{12}$1112​. Which value below is the best estimate of $\frac{11}{12}$1112​?

    $2$2

    A

    $0$0

    B

    $\frac{1}{2}$12​

    C

    $1$1

    D

  2. Now let’s look at $\frac{16}{33}$1633​. Which value below is the best estimate of $\frac{16}{33}$1633​?

    $\frac{1}{2}$12​

    A

    $0$0

    B

    $2$2

    C

    $1$1

    D

  3. Rewrite the fraction difference using the rounded values. Do not evaluate the difference.

  4. Calculate the fraction difference $1-\frac{1}{2}$1−12​.

  5. Is Bianca's estimate reasonable?

    No

    A

    Yes

    B

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