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Lesson 13 of 14

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Ten ideas, ten checks. The table sits first, so there is somewhere to look back.

Ten moves for a number too long to take in at a glance
MoveHow it goes
Name the columnThousand, ten thousand, lakh, ten lakh, crore. Each is ten of the one before.
Place the commasIndian: after 3 digits, then twos. International: threes all the way.
Read it outLabel each Indian group crore, lakh, thousand. Skip a group of zeros.
Compare twoMore digits wins. On a tie, walk left and stop at the first difference.
Round itHalf that place or more goes up. Less than half goes down.
Estimate firstRound both, multiply, and keep answers between half and double.
Feel the sizeA lakh is ten pages of 10,000 dots. A crore is a hundred lakh.
Round which wayRound up when you need enough. Round down when you must not go over. Keep the exact number when it has to match.
Lean one wayRound 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 timesTimes 5 is times 10, then half. Times 25 is times 100, then a quarter. Times 125 is times 1000, then an eighth.

One page holds 10,000 dots. How many pages hold one lakh dots?

  • 10
  • 100
  • 1,000

A note says 1,00,000 bricks. A bill says 100,000 bricks. What is the difference in the loads?

  • The bill is bigger, because its commas sit in different places
  • There is none — the same six digits, in two comma patterns
  • The note is bigger, because it carries an extra comma

How is 3,00,45,000 read out loud?

  • Three crore forty-five thousand
  • Three lakh forty-five thousand
  • Three crore forty-five lakh

Which is bigger, 99,99,999 or 1,00,00,000?

  • 99,99,999, because it starts with a 9
  • 1,00,00,000
  • They are equal, because both are just under a crore

What is 4,999 rounded to the nearest thousand?

  • 4,000, because you cross out the last three digits
  • 4,900, because you change the last two digits to 0
  • 5,000

A cart carries 1,480 bricks and 260 carts arrive. Someone claims the total is 38,48,000. Does that pass the size check?

  • Yes — it is close enough to be worth keeping
  • There is no way to tell without doing the exact multiplication
  • No. Round to 1,500 × 300 = 4,50,000. 38,48,000 is far above double that

Count one thing every second. A lakh takes a little over a day. Roughly how long does a crore take?

  • About ten days, because a crore is ten lakh
  • About 116 days, because a crore is a hundred lakh
  • About a year, because big numbers take a long time

A class of 52 students needs one chair each. Chairs come in stacks of 10. How many chairs should you order?

  • 60
  • 50, the nearest ten
  • 52, the exact count

You round 4,38,000 and 2,61,000 both up to a lakh. You add and get 8,00,000. What do you know about the real sum?

  • It is less than 8,00,000
  • It is more than 8,00,000
  • It is exactly 8,00,000

Which line gives 52 × 25 by the quick way?

  • Half of 52 is 26, then 26 × 100 = 2,600
  • 52 × 10 = 520, then half is 260
  • 52 × 100 = 5,200, then a quarter is 1,300

Part I, Chapter 1 of your textbook is Large Numbers Around Us. You have named the columns and moved the commas. You have drawn a lakh out in dots. Now read the chapter and try its questions.

Next: Arithmetic Expressions. Reading a calculation as a recipe. Finding out why the order you work in changes the answer.

Reveal every answer to finish this lesson.