CBSE Class 4 Mathematics Mental Arithmetic Set 02

QNO1: Round the numbers to nearest tens

a) 132
Answer: 130
Look at the digit in the ones place, which is 2.
Since 2 is less than 5, round down by keeping the tens digit the same and replacing the ones digit with 0.
Rounded value = 130
In simple words: When the last digit is less than 5, we round down to the nearest ten.
Exam Tip: Always look at the digit one place to the right of the place value you are rounding to.

b) 1567
Answer: 1570
Look at the digit in the ones place, which is 7.
Since 7 is greater than or equal to 5, round up by increasing the tens digit by 1 and replacing the ones digit with 0.
\(6 + 1 = 7\), so 1567 rounded to the nearest tens is 1570.
In simple words: When the last digit is 5 or more, we round up to the next ten.

Exam Tip: Remember to carry over correctly when rounding up causes a change in the tens column.

QNO2: Estimate the sum or the difference by rounding the numbers to nearest thousands.
a) 8452 + 2121
Answer: 10000
First, round each number to the nearest thousand:
For 8452, the digit in the hundreds place is 4 (less than 5), so it rounds down to 8000.
For 2121, the digit in the hundreds place is 1 (less than 5), so it rounds down to 2000.
Now, add the rounded numbers:
\(8000 + 2000 = 10000\)
In simple words: Round each number first, then perform the addition.

Exam Tip: Always round the individual numbers before calculating the estimated sum or difference.

b) 7896 - 2457
Answer: 6000
First, round each number to the nearest thousand:
For 7896, the digit in the hundreds place is 8 (greater than or equal to 5), so it rounds up to 8000.
For 2457, the digit in the hundreds place is 4 (less than 5), so it rounds down to 2000.
Now, subtract the rounded numbers:
\(8000 - 2000 = 6000\)
In simple words: Round both numbers to thousands first, then subtract.

Exam Tip: Check the hundreds digit carefully to decide whether to round up or down.

QNO3: Estimate the product or the quotient by rounding the numbers to nearest hundreds.
a) 178 \times 456
Answer: 100000
First, round each number to the nearest hundred:
For 178, the tens digit is 7 (greater than or equal to 5), so it rounds up to 200.
For 456, the tens digit is 5 (greater than or equal to 5), so it rounds up to 500.
Now, multiply the rounded numbers:
\(200 \times 500 = 100000\)
In simple words: Round the numbers to the nearest hundred first, then multiply.

Exam Tip: Count the total number of zeros when multiplying rounded multiples of 100.

b) 545 \div 121
Answer: 5
First, round each number to the nearest hundred:
For 545, the tens digit is 4 (less than 5), so it rounds down to 500.
For 121, the tens digit is 2 (less than 5), so it rounds down to 100.
Now, divide the rounded numbers:
\(500 \div 100 = 5\)
In simple words: Round both numbers to the nearest hundred, then divide.

QNO4: Circle the numbers that can be rounded to 600.
545, 624, 578, 650, 698
Answer: 624 and 578
Let us check each number by rounding to the nearest hundred:
- 545 rounds to 500
- 624 rounds to 600
- 578 rounds to 600
- 650 rounds to 700
- 698 rounds to 700
Therefore, the numbers that can be rounded to 600 are 624 and 578.
In simple words: Find numbers between 550 and 649, as they round off to 600.
Exam Tip: A number rounds to 600 if its value lies between 550 and 649 inclusive.