Chapter 1: Large Numbers Around Us
Lakhs, crores and billions are all around us: in populations, distances and record facts. In this chapter we read and write large numbers in the Indian and American systems, see how many ways a number can be built, round and estimate, and find shortcuts and patterns in big products.
In this chapter you will
- read and write numbers in lakhs and crores
- change between the Indian and American systems
- round to the nearest thousand, lakh or crore and estimate
- use shortcuts and predict the length of a product
Topics
- A Lakh Varieties! NCERT 1.1
1,00,000 (one lakh) is the smallest 6-digit number: 99,999 + 1.
- Land of Tens NCERT 1.2
A number can be built from +1, +10, +100… presses in many ways.
- Of Crores and Crores! NCERT 1.3
1 lakh = 5 zeros, 1 crore = 7 zeros, 1 arab = 9 zeros; 1 million = 6 zeros, 1 billion = 9 zeros.
- Exact and Approximate Values NCERT 1.4
Approximate values are often enough; some things (money, phone numbers, medicine) must be exact.
- Patterns in Products NCERT 1.5–1.6
× 5 = ÷ 2 then × 10; × 25 = ÷ 4 then × 100; × 125 = ÷ 8 then × 1000.
Key words
- Lakh
- 1,00,000: one hundred thousand
- Crore
- 1,00,00,000: one hundred lakh
- Million
- 1,000,000: ten lakh
- Billion
- 1,000,000,000: one hundred crore (one arab)
- Round off
- replace a number with a nearby simpler one
- Estimate
- a quick, close guess found by rounding
Summary
- 1 lakh = 1,00,000; 1 crore = 1,00,00,000; 1 arab = 100 crore.
- Indian commas 3-2-2-…; American 3-3-3-…; 10 lakh = 1 million, 100 crore = 1 billion.
- Fewest button clicks = sum of the digits; to round, look at the next digit (5 or more → up).
- × 25 = ÷ 4 × 100; an a-digit × b-digit product has a + b − 1 or a + b digits.