Chapter 1: Knowing Our Numbers

(a) 730 + 998

Round off to hundreds

730 rounds off to 700

998 rounds off to 1000

Hence, 730 + 998 = 700 + 1000 = 1700

(b) 796 – 314

Round off to hundreds

796 rounds off to 800

314 rounds off to 300

Hence, 796 – 314 = 800 – 300 = 500

(c) 12904 + 2888

Round off to thousands

12904 rounds off to 13000

2888 rounds off to 3000

Hence, 12904 + 2888 = 13000 + 3000 = 16000

(d) 28292 – 21496

Round off to thousands

28292 round off to 28000

21496 round off to 21000

Hence, 28292 – 21496 = 28000 – 21000 = 7000

Ten more such examples are

(i) 330 + 280 = 300 + 300 = 600

(ii) 3937 + 5990 = 4000 + 6000 = 10000

(iii) 6392 – 3772 = 6000 – 4000 = 2000

(iv) 5440 – 2972 = 5000 – 3000 = 2000

(v) 2175 + 1206 = 2000 + 1000 = 3000

(vi) 1110 – 1292 = 1000 – 1000 = 0

(vii) 910 + 575 = 900 + 600 = 1500

(viii) 6400 – 4900 = 6000 – 5000 = 1000

(ix) 3731 + 1300 = 4000 + 1000 = 5000

(x) 6485 – 4319 = 6000 – 4000 = 2000

(a) 439 + 334 + 4317

Rounding off to the nearest hundreds

439 + 334 + 4317 = 400 + 300 + 4300

= 5000

Rounding off to the nearest tens

439 + 334 + 4317 = 440 + 330 + 4320

= 5090

(b) 108734 – 47599

Rounding off to the nearest hundreds

108734 – 47599 = 108700 – 47600

= 61100

Rounding off to the nearest tens

108734 – 47599 = 108730 – 47600

= 61130

(c) 8325 – 491

Rounding off to the nearest hundreds

8325 – 491 = 8300 – 500

= 7800

Rounding off to the nearest tens

8325 – 491 = 8330 – 490

= 7840

(d) 489348 – 48365

Rounding off to the nearest hundreds

489348 – 48365 = 489300 – 48400

= 440900

Rounding off to the nearest tens

489348 – 48365 = 489350 – 48370

= 440980

Four more examples are as follows:

(i) 4853 + 662

Rounding off to the nearest hundreds

4853 + 662 = 4800 + 700

= 5500

Rounding off to the nearest tens

4853 + 662 = 4850 + 660

= 5510

(ii) 775 – 390

Rounding off to the nearest hundreds

775 – 390 = 800 – 400

= 400

Rounding off to the nearest tens

775 – 390 = 780 – 400

= 380

(iii) 6375 – 2875

Rounding off to the nearest hundreds

6375 – 2875 = 6400 – 2900

= 3500

Rounding off to the nearest tens

6375 – 2875 = 6380 – 2880

= 3500

(iv) 8246 – 6312

Rounding off to the nearest hundreds

8246 – 6312 = 8200 – 6300

= 1900

Rounding off to the nearest tens

8246 – 6312 = 8240 – 6310

= 1930

(a) 578 × 161

Rounding off by general rule

578 and 161 rounded off to 600 and 200, respectively

600

× 200

____________

120000

_____________

(b) 5281 × 3491

Rounding off by general rule

5281 and 3491 rounded off to 5000 and 3500, respectively

5000

× 3500

_________

17500000

_________

(c) 1291 × 592

Rounding off by general rule

1291 and 592 rounded off to 1300 and 600, respectively

1300

× 600

_____________

780000

______________

(d) 9250 × 29

Rounding off by general rule

9250 and 29 rounded off to 9000 and 30, respectively

9000

× 30

_____________

270000

______________

Disclaimer:

Dropped Topics – 1.3.1 Estimation, 1.3.2 Estimating to the nearest tens by rounding off, 1.3.3 Estimating to the nearest hundreds by rounding off, 1.3.4 Estimating to the nearest thousands by rounding off, 1.3.5 Estimating outcomes of number situations, 1.3.6 To estimate sum or difference, 1.3.7 To estimate products, 1.4 Using brackets, 1.4.1 Expanding brackets, 1.5 Roman numerals.