CBSE Class 4 Mathematics Mental Arithmetic Set 01

QNO1: Round the numbers to nearest tens

a) 245
Answer: Look at the digits of 245.
The digit in the tens place is 4, and the digit in the units place is 5.
Since the units digit is 5 or more, we round up the tens digit by 1.
Rounded to the nearest tens = 250
In simple words: When rounding to the nearest ten, if the last digit is 5 or more, increase the tens digit by 1 and make the last digit 0.

Exam Tip: Always look at the digit to the immediate right of the place value you are rounding to.

b) 1221
Answer: Look at the digits of 1221.
The digit in the tens place is 2, and the digit in the units place is 1.
Since the units digit (1) is less than 5, keep the tens digit the same and replace the units digit with 0.
Rounded to the nearest tens = 1220
In simple words: If the last digit is less than 5, keep the tens digit unchanged and change the last digit to 0.

Exam Tip: Check only the units digit when rounding to the nearest tens.

QNO2: Estimate the sum or the difference by rounding the numbers to nearest thousands.

a) 6472 + 1896
Answer: Round each number to the nearest thousands first.
For 6472: The hundreds digit is 4 (less than 5), so it rounds down to 6000.
For 1896: The hundreds digit is 8 (5 or more), so it rounds up to 2000.
Now, add the estimated values:
\(6000 + 2000 = 8000\)
In simple words: Round each number to the nearest thousand first, and then perform the addition.

Exam Tip: Always round the numbers individually before performing the operation.

b) 3672 - 1876
Answer: Round each number to the nearest thousands first.
For 3672: The hundreds digit is 6 (5 or more), so it rounds up to 4000.
For 1876: The hundreds digit is 8 (5 or more), so it rounds up to 2000.
Now, subtract the estimated values:
\(4000 - 2000 = 2000\)
In simple words: Round both numbers to the nearest thousand and then subtract.

QNO3: Estimate the product or the quotient by rounding the numbers to nearest hundreds.

a) 192 \times 326
Answer: Round each number to the nearest hundreds first.
For 192: The tens digit is 9 (5 or more), so it rounds up to 200.
For 326: The tens digit is 2 (less than 5), so it rounds down to 300.
Now, multiply the estimated values:
\(200 \times 300 = 60000\)
In simple words: Round both numbers to the nearest hundred and multiply them.
Exam Tip: When multiplying numbers with zeros, multiply the non-zero digits first and then attach all the zeros together.

b) 836 \div 421
Answer: Round each number to the nearest hundreds first.
For 836: The tens digit is 3 (less than 5), so it rounds down to 800.
For 421: The tens digit is 2 (less than 5), so it rounds down to 400.
Now, divide the estimated values:
\(800 \div 400 = 2\)
In simple words: Round both numbers to the nearest hundred and then divide.

Exam Tip: Estimating makes division easier by simplifying numbers into compatible multiples.

QNO4: Circle the numbers that can be rounded to 300.
298, 326, 402, 230, 199
Answer: A number rounds to 300 to the nearest hundred if its value lies between 250 and 349.
Let us check each number:
- 298: Rounds to 300 (Correct)
- 326: Rounds to 300 (Correct)
- 402: Rounds to 400 (Incorrect)
- 230: Rounds to 200 (Incorrect)
- 199: Rounds to 200 (Incorrect)
Numbers that can be rounded to 300 are 298 and 326.
In simple words: Check which numbers fall in the range from 250 to 349.

Exam Tip: The range for rounding to any hundred spans from 50 below the target hundred up to 49 above it.