The big picture
Here are the ten ideas, in the order you learned them.
| Idea | Remember |
|---|---|
| Past the thousands | Thousand, ten thousand, lakh, ten lakh, crore. Each is ten of the one before. |
| Two groupings | Indian steps in twos after 3 digits. International steps in threes. |
| Reading it aloud | Label each Indian group crore, lakh, thousand. Zeros say nothing. |
| Which one is bigger | More digits wins. On a tie, walk left and stop at the first difference. |
| Rounding | Half that place or more goes up. Less than half goes down. |
| Estimate first | Round both, multiply, then keep answers between half and double. |
| How big is that | A lakh is ten pages of 10,000 dots, or about a day of counting. |
| Round which way | Round up when you need enough. Round down when you must not go over. Keep the exact number when it has to match. |
| One-sided estimates | Round both numbers down: the estimate is never more than the sum. Round both up: it is never less than the sum. For a difference, round the first down and the second up. The estimate is never more than the difference. |
| Quick times | Times 5 is times 10, then half. Times 25 is times 100, then a quarter. Times 125 is times 1000, then an eighth. |
Part I, Chapter 1 is Large Numbers Around Us. You have counted past the thousand and placed commas two ways. You have drawn a lakh. Now see what the chapter does next.
A large number is more than a long row of digits. It has columns you can name. You can round it to a place. You can draw its size.