Chapter 2.4 (Mathematics 5)

Estimating sums by rounding off addends

Rounding off numbers to the nearest hundreds

Group Activity 4

In groups of five:

  1. Show the numbers 300 and 400 on a number line shown.
  1. Mark the number 350 half way between 300 and 400.
  2. Show some numbers between:
    1. 300 and 350
    2. 350 and 400
  3. List three numbers nearer to 300 than 400.
  4. List three numbers nearer to 400 than 300.
  5. Discuss and share the two lists of numbers with other groups.

Note

The number 350 is considered to be nearer to 400.

Example 4

By rounding off each number to the nearest hundred, find the estimate sum of 1 343 and 791.

  • 1 343 to the nearest hundred is 1 300.
  • 791 rounded off to the nearest hundred is 800.
  • 1 300 + 800 = 2 100.

Therefore, estimate sum is 2 100.

Exercise D

          Digital Time

          Rounding off to the nearest hundreds
          • 200
          • 1 500
          • 3 500
          • 2 600
          • 49 200
          • 942 300
          • 69 + 142
          • 345 + 1 235
          • 2 523 + 950
          • 1 928 + 729
          • 41 390 + 7 779
          • 265 199 + 915 781

          D3

          Pupils from two schools planted 3 728 and 3 251 tree seedlings. Round off each number to the nearest hundreds and work out the estimate sum of the number of seedlings planted.

          D4

          A milk processor sold 8 255 packets of fresh milk and 3 590 packets of sour milk. Round off each number to the nearest hundreds and find the estimate sum of the number of packets of milk sold.

          D5

          There were 16 375 men and 17 540 women in a town. Find the estimate sum of the number of men and women by rounding off each number to the nearest hundred.

          Rounding off numbers to the nearest thousands

          Group Activity 5

          In groups:

          1. Draw a number line and mark as shown.
          1. Write five numbers between 3 000 and 3 500.
          2. Write five numbers between 3 500 and 4 000.
          3. Discuss the questions below:
            1. Which of the numbers are nearer to 3 000 than to 4 000?
            2. Which of the numbers are nearer to 4 000 than to 3 000?
          4. Discuss rounding off numbers to the nearest thousands and share with other groups.

          Example 5

          In a town 9 592 boys and 8 385 girls took part in a litter collection exercise. By rounding off each number to the nearest thousand, find the estimate sum of the number of participants.

          • Numbers of boys to the nearest thousand is 10 000.
          • Numbers of girls to the nearest thousand is 8 000.

          Estimate sum of the participants is 18 000.

          Exercise E

          1. 5 090 
          2. 5 701 
          3. 6 485 
          4. 6 509 
          1. 1 085 + 750
            Estimate sum is .
          2. 908 + 2 455
            Estimate sum is .
          3. 890 + 1 650
            Estimate sum is .
          4. 312 399 + 215 900
            Estimate sum is .
          5. 410 512 + 19 605
            Estimate sum is .
          6. 320 250 + 69 555
            Estimate sum is .

          E3

          A farmer sold 1 080 watermelons and 857 pineapples. Round off the numbers to the nearest thousands and work out the estimate sum of the number of fruits.

          E4

          The number of people who attended two HIV screening camps were 21 097 and 18 635. Round off the numbers to the nearest thousands. Work out the estimate sum.