The Cleanest Village
Using a round number
Use a round number to add quickly in your head.
In your book, Maths Mela (Chapter 7), some sums can be done quickly in your head, without writing columns. The trick is to use a nearby round number, like 10, 20 or 100.
The book shows 410 − 12. Think of 12 as 10 and 2 more. 410 − 10 is 400. Then take away 2 more: 2 less than 400 is 398. So 410 − 12 = 398.
To add 99, add 100 and then take away 1, because 99 is 1 less than 100. 811 + 100 = 911, and 1 less is 910. So 811 + 99 = 910.
To take away 19, take away 20 and then add 1 back, because you took away 1 too many. 945 − 20 = 925, and 1 more is 926. So 945 − 19 = 926. Some sums are quick already: 675 + 5 makes the round number 680, and 204 − 10 = 194.
Practise this lesson
Questions that get harder as you improve, a Tutor that helps without giving the answer away, and progress you can see. Free for every learner.
No account, no email address. Opens the real product.