For rounding off to the nearest tens let us look at the number ray given below.
Consider the numbers 12 and 19.
When we plot these numbers on a number line we observe that 12 lies between 10 and 20. Also 12 is nearest to 10 than 20. So, we round off 12 as 10, correct to the nearest tens.
If we consider 19, we observe that it is nearer to 20 than 10. So, we round off 19 to 20, correct to the nearest tens.
Now, let us consider the number 15.
If we plot this number on number line we find that 15 lies half-way between 10 and 20. By convention, we round off 15 to 20.
Now we will learn how to estimate the number 23, 27 and 29.
Look at the arrow at 23. It is nearer to 20 than 30 because 23 - 20 = 3 while 30 – 23 = 7. We take 23 as 20 when we estimate it to the nearest 10.
Again, look at the arrow at 27. The arrow at 27 is nearer to 30 than 20 because 30 – 27 = 3 and 27 – 20 = 7. So, 27 is taken as 30.
In the same way, look at the arrow at 29. It is nearer to 30 than 20 because 30 - 29 = 1 while 29 – 20 = 9. So, 29 is taken as 30 since it is near to 30 than 20.
How will we round off the number 25?
25 is equal distance from 20 and 30. By convention it is taken as 30.
1. Round off to the nearest tens:
(i) 362
The given number is 362.
Its ones or unit digit is 2, which is less than 5. So, we replace the ones digit by 0 to get the rounded off number.
Hence, rounded off number = 360.
(ii) 909
The given number is 909.
Its ones or unit digit is 9, which is greater than 5. So, we increase the tens digit by 1 and replace the ones digit by 0 to get the rounded off number.
Hence, rounded off number = 910.
2. How to estimate 2534 to the nearest 10?
34 is nearer to 30 than 40, so 2534 is taken as 2530.
From the above examples, to estimate to nearest tens we can generalize that,
(i) the numbers having 1, 2, 3, 4 at ones or units place are rounded off downwards.
(ii) the numbers having 5, 6, 7, 8, 9 at ones or units place are rounded off upwards.
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