Chapter - 1 Knowing Our Numbers

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Knowing Our Numbers

Question:

1. Estimate each of the following using general rule:

(a) 730 + 998 (b) 796 – 314 (c) 12,904 + 2,888 (d) 28,292 – 21,496

Make ten more such examples of addition, subtraction and estimation of their outcome.

 

Solution: (a) 730+ 998

Round off to hundreds

730 rounds off = 700

998 rounds off to = 1000

Estimated sum = 1700

Hence, by estimate 730 + 998 = 700 + 1000 =1700

 

(b) 796 - 314

Round off to hundreds

796 rounds off to = 800

314 rounds off to = 300

Estimated difference = 500

Hence, by estimate 796 - 314 = 800 - 300 = 500

 

(c) 12904 + 2888

Round off to thousands

12904 rounds off to = 13000

2888 rounds off to = 3000

Estimated sum = 16000

Hence, by estimate 12904 + 2888 = 13000 + 3000 = 16000

 

d) 28292 - 21496

Round off to thousands

28292 round off to = 28000

21496 round off to = 21000

Estimated difference  = 7000

Hence, by estimate 28292 - 21496=28000 - 21000 = 7000

 

2. Give a rough estimate (by rounding off to nearest hundreds) and also a closer estimate (by rounding off to nearest tens) :

(a) 439 + 334 + 4,317 (b) 1,08,734 – 47,599 (c) 8325 – 491 (d) 4,89,348 – 48,365

Make four more such examples.

 

Solution: (a) 439+334+4317

Rounding off to nearest hundreds

439 round off to = 400

334 round off to = 300

4317 round off to = 4300

Estimated sum = 400 + 300 + 4300 = 5000

 

Rounding off to nearest tens

439 round off to = 440

334 round off to = 330

4317 round off to = 4320

Estimated sum = 440+ 330 + 4320 = 5090

 

(b) 108734 - 47599

Rounding off to nearest hundreds

108734 round off to = 108700

47599 round off to = 47600

Estimated difference = 108700 - 47600 = 61100

 

Rounding off to nearest tens

108734 round off to = 108730

47599 round off to = 47600

Estimated difference = 108730 - 47600 = 61130

 

(c) 8325 – 491

Rounding off to nearest hundreds

8325 round off to = 8300

491 round off to = 500

Estimated difference = 8300 - 500 = 7800

 

Rounding off to nearest tens

8325 round off to = 8330

491 round off to = 490

Estimated difference = 8330 - 490 = 7840

 

(d) 489348 - 48365

Rounding off to nearest hundreds

489348 round off to = 489300

48365 round off to = 48400

Estimated difference = 489300 - 48400 = 440900

 

Rounding off to nearest tens

489348 round off to = 489350

48365 round off to = 48370

Estimated difference = 489350 - 48370 = 440980

 

3. Estimate the following products using general rule:

(a) 578 × 161 (b) 5281 × 3491 (c) 1291 × 592 (d) 9250 × 29

Make four more such examples.

 

Solution: (a) 578 x 161

578 rounds off to = 600

161 rounds off to = 200

Product = 600 x 200 = 1,20,000

 

(b) 5281 x 3491

5281 rounds of to = 5,000

3491 rounds off to = 3,500

Product = 5,000 x 3,500 = 1,75,00,000

 

(c) 1291 × 592

1291 rounds off to = 1300

592 rounds off to = 600

Product = 1300 x 600 = 7,80,000

 

(d) 9250 x 29

9250 rounds off to = 10,000

29 rounds off to = 30

Product = 10,000 x 30 = 3,00.000

Answer:


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