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Answer Key & Detailed Solutions
Q1. [1 Mark]
Ninety crore eight lakh sixty thousand twelve
= 90,00,00,000 + 8,00,000 + 60,000 + 12
= 90,08,60,012
Q2. [1 Mark]
1 crore = 1,00,00,000
1 ten-thousand = 10,000
1,00,00,000 ÷ 10,000 = 1,000 ten-thousands
Q3. [1 Mark]
3,78,45,902
Positions: 3(Cr) 7(TL) 8(L) 4(TTh) 5(Th) 9(H) 0(T) 2(O)
The digit 8 is in the lakhs place.
Face value of 8 = 8
Place value of 8 = 8,00,000 (Eight lakh)
Q4. [1 Mark]
99,99,999 is a 7-digit number.
1,00,00,000 is an 8-digit number.
1,00,00,000 is greater.
Difference = 1,00,00,000 − 99,99,999 = 1
Q5. [1 Mark]
456 × 104 = 456 × 10,000 = 45,60,000
(Add 4 zeros to 456)
Q6. [1 Mark]
3,45,67,891
Crores digit: 3. Ten-lakhs digit: 4. Since 4 < 5, round down.
Answer: 3,00,00,000 (Three crore)
Q7. [2 Marks]
(a) 270546083 [1 mark]
Indian: 27,05,46,083 (Twenty-seven crore five lakh forty-six thousand eighty-three)
International: 270,546,083 (Two hundred seventy million five hundred forty-six thousand eighty-three)
(b) 509008400 [1 mark]
Indian: 50,90,08,400 (Fifty crore ninety lakh eight thousand four hundred)
International: 509,008,400 (Five hundred nine million eight thousand four hundred)
Q8. [2 Marks]
(a) 99,99,999: [1 mark]
Successor = 99,99,999 + 1 = 1,00,00,000
Predecessor = 99,99,999 − 1 = 99,99,998
(b) 10,00,00,000: [1 mark]
Successor = 10,00,00,000 + 1 = 10,00,00,001
Predecessor = 10,00,00,000 − 1 = 9,99,99,999
Q9. [2 Marks]
(a) 568 × 999 [1 mark]
Trick: 999 = 1000 − 1
568 × 999 = 568 × 1000 − 568 × 1
= 5,68,000 − 568 = 5,67,432
(b) 784 × 101 [1 mark]
Trick: 101 = 100 + 1
784 × 101 = 784 × 100 + 784 × 1
= 78,400 + 784 = 79,184
Q10. [2 Marks]
Height of one building = 18 × 3.5 = 63 metres [1 mark]
Height of Statue of Unity = 182 metres
Number of buildings = 182 ÷ 63 = 2.889... ≈ 3 buildings [1 mark]
(Need 3 buildings since 2 buildings = 126m, which is less than 182m)
Q11. [2 Marks]
(a) 45 million = 45 × 10 lakh = 450 lakh (= 4,50,00,000) [1 mark]
(b) 3.7 million = 3.7 × 10 lakh = 37 lakh (= 37,00,000) [1 mark]
Q12. [3 Marks]
(a) Winner: [1 mark]
B got most votes (5,12,045). Runner-up is D (4,63,580).
Winning margin = 5,12,045 − 4,63,580 = 48,465 votes
(b) Total votes: [1 mark]
3,48,729 + 5,12,045 + 2,97,816 + 4,63,580 + 1,85,230
= 18,07,400 (Eighteen lakh seven thousand four hundred)
(c) Winner rounded to nearest ten thousand: [1 mark]
5,12,045 → thousands digit is 2, which is < 5. Round down.
Answer: 5,10,000
Q13. [3 Marks]
(a) Pages per day: [1 mark]
8,500 × 24 = 2,04,000 pages
(b) Pages per year: [1 mark]
2,04,000 × 365 = 7,44,60,000 pages
= Seven crore forty-four lakh sixty thousand
(c) Weight in tonnes: [1 mark]
Total weight = 7,44,60,000 × 5 = 37,23,00,000 grams
In tonnes = 37,23,00,000 ÷ 10,00,000 = 3,723 tonnes
= Three thousand seven hundred twenty-three tonnes
Q14. [3 Marks]
(a) 673 × 4,891: [1½ marks]
Estimation: 700 × 5,000 = 35,00,000
Actual: 673 × 4,891 = 32,91,643
Error = 35,00,000 − 32,91,643 = 2,08,357 (6.3% error)
(b) 8,025 × 396: [1½ marks]
Estimation: 8,000 × 400 = 32,00,000
Actual: 8,025 × 396 = 31,77,900
Error = 32,00,000 − 31,77,900 = 22,100 (0.7% error)
Answer: (b) was more accurate because the rounding errors cancelled each other (8,025 rounded down, 396 rounded up).
Q15. [3 Marks]
Digits: 0, 1, 2, 3, 5, 6, 8, 9
Largest 8-digit: 9,86,53,210 [½ mark]
= Nine crore eighty-six lakh fifty-three thousand two hundred ten
Smallest 8-digit: 1,02,35,689 [½ mark]
= One crore two lakh thirty-five thousand six hundred eighty-nine
(b) Difference: [1 mark]
9,86,53,210 − 1,02,35,689 = 8,84,17,521
(c) Place value of 5: [1 mark]
In 9,86,53,210: 5 is in ten-thousands place = 50,000
In 1,02,35,689: 5 is in thousands place = 5,000
Sum = 50,000 + 5,000 = 55,000
Q16. [4 Marks]
(a) Days for Delhi-Chennai round trip: [1 mark]
Round trip = 2,180 × 2 = 4,360 km
Days = 4,360 ÷ 350 = 12.46 ≈ 13 days
(b) Yearly distance: [1 mark]
350 × 300 = 1,05,000 km
(c) In words: [½ mark]
One lakh five thousand kilometres
(d) Fuel cost: [1½ marks]
Litres needed = 1,05,000 ÷ 5 = 21,000 litres
Cost = 21,000 × 95 = ₹19,95,000
= Nineteen lakh ninety-five thousand rupees
Q17. [4 Marks]
(a) Monthly collection: [1 mark]
2,400 × 15 = ₹36,000
(b) Annual collection (10 months): [1 mark]
36,000 × 10 = ₹3,60,000
(c) Number of trees: [1 mark]
3,60,000 ÷ 450 = 800 trees
(d) CO2 absorbed: [1 mark]
800 × 22 = 17,600 kg
In tonnes = 17,600 ÷ 1,000 = 17.6 tonnes of CO2
Q18. Bonus [4 Marks]
Rule: If a number has m digits and another has n digits, their product has at least (m + n − 1) and at most (m + n) digits. [1 mark for rule]
(a) 999 × 99: m=3, n=2 → 4 to 5 digits [½ mark]
Actual: 999 × 99 = 98,901 → 5 digits ✅
(b) 1001 × 101: m=4, n=3 → 6 to 7 digits [½ mark]
Actual: 1001 × 101 = 1,01,101 → 6 digits ✅
(c) 50000 × 200: m=5, n=3 → 7 to 8 digits [½ mark]
Actual: 50,000 × 200 = 1,00,00,000 → 8 digits ✅
(d) 12345 × 6789: m=5, n=4 → 8 to 9 digits [½ mark]
Actual: 12,345 × 6,789 = 8,38,10,205 → 8 digits ✅
(Any two verifications with full computation earn the marks)