RBSE Solutions Class 5 Maths Chapter 17 Mental Mathematics More Ques

Practice

Question 1. Identify the right answer.
(i) Find the sum of a number between 406 and 496, and a number between 30 and 39.
(a) 483
(b) 683
(c) 883
(ii) Find the sum of a number between 390 and 399, and a number between 102 and 192.
(a) 495
(b) 345
(c) 265
Answer:
(i) The first number is of the form \( 4\_6 \) (between 406 and 496) and the second number is of the form \( 3\_ \) (between 30 and 39). When we estimate the sum, it will be between \( 406 + 30 = 436 \) and \( 496 + 39 = 535 \). From the given options, only 483 falls within this range. Estimation helps us quickly find possible answers. So, 483 is the correct answer.
(ii) The first number is of the form \( 39\_ \) (between 390 and 399) and the second number is of the form \( 1\_2 \) (between 102 and 192). When we estimate the sum, it will be between \( 390 + 102 = 492 \) and \( 399 + 192 = 591 \). From the given options, only 495 falls within this range. This technique is useful for quick mental calculations.
In simple words: For part (i), the sum is around 436 to 535, so 483 is correct. For part (ii), the sum is around 492 to 591, so 495 is correct.

🎯 Exam Tip: When dealing with ranges, always calculate the minimum and maximum possible values of the sum or difference to narrow down the correct option.

Solve And Find Correct Option.
What is the result of subtracting a number between 205 and 294 from a number between 750 and 759?
(a) 572
(b) 613
(c) 512
Answer: (c) 512
The first number is between 750 and 759 (e.g., \( 75\_ \)). The second number is between 205 and 294 (e.g., \( 2\_5 \)). To find the approximate range of the difference, we subtract the largest possible second number from the smallest possible first number for the lower bound, and the smallest possible second number from the largest possible first number for the upper bound. So, the difference will be between \( 750 - 294 = 456 \) and \( 759 - 205 = 554 \). Among the given options, only 512 falls within this range.
In simple words: The first number is about 750, and the second is about 200 to 290. So, their difference will be roughly between 456 and 554. Only 512 fits this range.

🎯 Exam Tip: For subtraction estimation, remember that the smallest possible answer comes from subtracting the largest number from the smallest number, and vice-versa for the largest answer.

Question 3. Find out the digit which comes in place of in following questions.
(a) \( 482 + 2\_6 = 738 \)
(b) \( 652 - 3\_7 = 305 \)
Answer:
(a) To find the missing digit in \( 482 + 2\_6 = 738 \):
First, add the units digits: \( 2 + 6 = 8 \). This is correct.
Next, add the tens digits: We need \( 8 + \text{missing digit} \) to result in a number ending in 3 (from 738). This means \( 8 + \text{missing digit} = 13 \). So, the missing digit is \( 13 - 8 = 5 \). We carry over 1 to the hundreds place.
Finally, add the hundreds digits: \( 4 + 2 + 1 (\text{carry}) = 7 \). This matches 738.
Therefore, the missing digit is 5. The full sum is:
\[ \begin{array}{r} 482 \\ +\ 256 \\ \hline 738 \end{array} \]
(b) To find the missing digit in \( 652 - 3\_7 = 305 \):
First, subtract the units digits: \( 2 - 7 \). Since 2 is smaller than 7, we borrow 1 from the tens place (5 becomes 4). So, \( 12 - 7 = 5 \). This is correct.
Next, subtract the tens digits: We now have 4 in the tens place of the top number. We need \( 4 - \text{missing digit} = 0 \). This means the missing digit is 4.
Finally, subtract the hundreds digits: \( 6 - 3 = 3 \). This matches 305.
Therefore, the missing digit is 4. The full subtraction is:
\[ \begin{array}{r} 652 \\ -\ 347 \\ \hline 305 \end{array} \]
In simple words: For (a), by adding in columns, the missing tens digit is 5. For (b), by subtracting in columns and borrowing, the missing tens digit is 4.

🎯 Exam Tip: When finding missing digits in addition or subtraction, always start with the units column and work your way to the left, carefully handling carries and borrows.