Playing with Digits

Class 6 Maths · Chapter 3: Number Play · NCERT Ganita Prakash section 3.4

Learn

There are 9 one-digit numbers (1 to 9). From 10 to 99 there are 90 two-digit numbers, then 900 three-digit numbers. Each time, 10 times more!

The digit sum of a number adds up its digits. Kabir found that 77, 158 and 914 all have digit sum 14.

7 + 7 = 1 + 5 + 8 = 9 + 1 + 4 = 14

The smallest number with digit sum 14 is 59: use as few digits as you can, with 9s on the right. The largest 5-digit number with digit sum 14 is 95,000.

Digit detectives: from 1 to 100, the digit 7 is written 20 times: 10 times in the ones place (7, 17 … 97) and 10 times in the tens place (70 to 79).

10 + 10 = 20

Explore: turn the digit wheels and watch the digit sum.

1. Make a 2-digit number whose digit sum is 14.
2. Now make the smallest number with digit sum 14.
3. Make the largest 3-digit number with digit sum 14.

Remember

• 9 one-digit, 90 two-digit, 900 three-digit numbers …
• Digit sum: add the digits. Smallest number for a digit sum: 9s on the right.

Quick check

What is the digit sum of 3,705?

1. 12
2. 14
3. 15
4. 3,705

Think about it

Find the digit sums of 345, 456 and 567 (digits next to each other). What pattern do you see?

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.