Hinglish

Mental Math

Class 6 Maths · Chapter 3: Number Play · NCERT Ganita Prakash section 3.8

Learn

Big sums are easy in your head with round numbers. Using the numbers in the boxes as many times as you like: 5,300 = 2,500 + 2,500 + 300.

5,300 = 2,500 + 2,500 + 300

Another one: 31,600 = 20,000 + 11,000 + 300 + 300.

20,000 + 11,000 + 600 = 31,600

We can also take away: 49,500 = 50,000 − 900 + 400.

50,000 − 900 + 400 = 49,500

Always, sometimes or never? A 3-digit number + a 3-digit number can give 3 digits (120 + 340 = 460) or 4 digits (700 + 800 = 1,500). So the statement '3-digit + 3-digit gives a 3-digit number' is sometimes true.

Explore: make big numbers in your head using number cards. A card can be used again.

  1. Make 5,300.
  2. Make 31,600.
  3. Make 49,500. You can take away too.

Remember

  • Break big numbers into round parts to add and subtract in your head.
  • Test 'always, sometimes, never' with the smallest and biggest numbers of each kind.

Quick check

40,000 − 800 + 300 = ?

  1. 39,500
  2. 39,800
  3. 41,100
  4. 39,200
Answer

39,500. 40,000 − 800 = 39,200, then + 300 = 39,500.

Think about it

Can you make exactly 1,000 by adding only 2,500s, 300s and 20,000s?

One answer

No. 2,500 and 20,000 are already bigger than 1,000, and 300s make 300, 600, 900, then 1,200: they jump over 1,000.

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.