Q1. Estimate each of the following using general rule:
Make ten more such examples of addition, subtraction and estimation of their outcome.
(a) 730 + 998
Solutions:
730 + 998
Round off to hundreds
730 is rounds off to 700
998 is rounds off to 1000
Hence, 730 + 998 = 700 + 1000 = 1700
(b) 796 – 314
Solution:
796 – 314
Round off to hundreds
796 is rounds off to 800
314 is rounds off to 300
Hence, 796 – 314 = 800 – 300 = 500
(c) 12904 + 2888
Solution:
12904 + 2888
Round off to thousands
12904 is rounds off to 13000
2888 is rounds off to 3000
Hence, 12904 + 2888 = 13000 + 3000 = 16000
(d) 28292 – 21496
Solution:
28292 – 21496
Round off to thousands
28292 is round off to 28000
21496 is round off to 21000
Hence, 28292 – 21496 = 28000 – 21000 = 7000
Ten more such examples are
(i) 333 + 286 = 300 + 300 = 600
(ii) 3938 + 5995 = 4000 + 6000 = 10000
(iii) 6391 – 3773 = 6000 – 4000 = 2000
(iv) 5434 – 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
2. Give a rough estimate (by rounding off to nearest hundreds) and also a closer estimate (by rounding off to nearest tens):
Make four more such examples.
(a) 439 + 334 + 4317
Solutions:
439 + 334 + 4317
Rounding off to nearest hundreds
439 + 334 + 4317 = 400 + 300 + 4300
= 5000
Rounding off to nearest tens
439 + 334 + 4317 = 440 + 330 + 4320
= 5090
(b) 108734 – 47599
Solution:
108734 – 47599
Rounding off to nearest hundreds
108734 – 47599 = 108700 – 47600
= 61100
Rounding off to nearest tens
108734 – 47599 = 108730 – 47600
= 61130
(c) 8325 – 491
Solution:
8325 – 491
Rounding off to nearest hundreds
8325 – 491 = 8300 – 500
= 7800
Rounding off to nearest tens
8325 – 491 = 8330 – 490
= 7840
(d) 489348 – 48365
Solution:
489348 – 48365
Rounding off to nearest hundreds
489348 – 48365 = 489300 – 48400
= 440900
Rounding off to nearest tens
489348 – 48365 = 489350 – 48370
= 440980
Four more examples are as follows
(i) 4854 + 661
Rounding off to nearest hundreds
4854 + 661 = 4800 + 700
= 5500
Rounding off to nearest tens
4854 + 661 = 4850 + 660
= 5510
(ii) 774 – 390
Rounding off to nearest hundreds
774 – 390 = 800 – 400
= 400
Rounding off to nearest tens
774 – 390 = 780 – 400
380
(iii) 6374 – 2876
Rounding off to nearest hundreds
6374 – 2876 = 6400 – 2900
= 3500
Rounding off to nearest tens
6374 – 2876 = 6380 – 2880
3500
(iv) 8245 – 6313
Rounding off to nearest hundreds
8245 – 6313 = 8200 – 6300
1900
Rounding off to nearest tens
8245 – 6313 = 8240 – 6310
= 1930
Q3. Estimate the following products using general rule:
Make four more such examples.
(a) 578 * 161
Solutions:
578 * 161
Rounding off by general rule
598 and 161 rounded off to 600 and 200 respectively
600*200 = 120000
(b) 5281 * 3491
Solution:
5281 * 3491
Rounding off by general rule
5281 and 3491 rounded off to 5000 and 3500 respectively
5000*3500 = 17500000
(c) 1291 * 592
Solution:
1291 * 592
Rounding off by general rule
1291 and 592 rounded off to 1300 and 600 respectively
1300 * 600 = 780000
(d) 9250 * 29
Solution:
9250 * 29
Rounding off by general rule
9250 and 29 rounded off to 9000 and 30 respectively
9000 * 30 = 270000
Four more examples are as follows
(i)199 * 29
Rounding off by general rule
199 and 29 rounded off to 200 and 30 respectively
200 * 30 = 6000
(ii) 398 * 30
Rounding off by general rule
400 and 30 is rounded off to 400 and 30 respectively
400 * 30 = 12000
(iii)556 * 46
Rounding off by general rule
556 and 46 is rounded off to 560 and 50 respectively
560 * 50 = 28000
