Tag: mental additions and subtractions

Questions Related to mental additions and subtractions

A 3-digit number 4a3 is added to another 3-digit number 984 to give the four-digit number 13b7 which is divisible by 11 Then (a + b) is

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 10

  2. 11

  3. 12

  4. 15

Show answer
Correct Option: A
Explanation:

According to the question
$4 a 3 + 9 8 4 = 1 3 b 7$
$a + 8 = b$
Then$b - a = 8$
According to the question  13b7 is divisible by 11
$(7 + 3) - (b + 1) = (9 - b)$
$(9 - b) = 0$
$b = 9$
$b = 9   and   a = 1$
Then $a+b=9+1=10$

In an examination, a student scores 4 marks for every correct answer and loses 1 mark for every wrong answer. If she attempts in all 50 questions and scores 120 marks, the number of questions she attempts correct is........

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 62

  2. 44

  3. 42

  4. 34

Show answer
Correct Option: D
Explanation:

Let student get x correct answer then 50-x student get wrong answer 
Then 4(x)+(50-x)(-1)=120
Or 4x-50+x=120
Or 5x=120+50=170
Or x=34

What is the value of $(\sqrt 7+\sqrt 5)(\sqrt 7-\sqrt 5)$?

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 1

  2. 2

  3. 5

  4. 3

Show answer
Correct Option: B
Explanation:

$(\sqrt 7+\sqrt 5)(\sqrt 7-\sqrt 5)$
$\Rightarrow \left(\sqrt {7}\right)^2-\left(\sqrt{5}\right)^2$
$\Rightarrow 7-5 =2$

If the sum of two consecutive odd numbers is 2004, then the smaller of the two numbers could be

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 2001

  2. 1001

  3. 1003

  4. 1

Show answer
Correct Option: B
Explanation:

Let the two consecutive odd numbers are x and x+2
$x+x+2 = 2004$
$x+x+2-2 = 2004 -2 $ (Subtract 2 from both the sides)
$2x = 2002$
$\frac{2x}{2} = \frac{2002}{2}$ (Divide 2 from both the sides)
$x = 1001$
Hence two numbers are 1001 and 1003

How many times does the sum of 375 and 125 contain their difference?

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. $5$

  2. $4$

  3. $3$

  4. $2$

Show answer
Correct Option: D
Explanation:

The sum of  375 and 125 $=375+125 = 500$
The difference of 375 and 125 $ = 375-125 = 250$
Hence Sum contains $500 \div 250 = 2 $ times their differences.

A man can row 16 km downstream in one hour but upstream he takes twice the time then the speed of the current is

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 8 km/h

  2. 12 km/h

  3. 4 km/h

  4. 2 km/h

Show answer
Correct Option: C
Explanation:

The man down stream is 16 km in one hour and he take twice time in upstream 

Then he take time 2 hour in 16 km 
So speed in upstream =$\frac{16}{2}=8 km /hr$
Then current spread =$\frac{16-8}{2}=4$ km/hr 

If a=2, b=3, c=4, then the difference between $\displaystyle 2\frac{3}{4}$ and $\displaystyle b\frac{a}{c}$ is

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. $-1/4$

  2. $1/4$

  3. $-3/4$

  4. $5/4$

Show answer
Correct Option: C
Explanation:

If a=2 ,b=3 and c=4

Then $b\tfrac{a}{c}=3\tfrac{2}{4}=3\tfrac{1}{2}$
Then difference between=$2\tfrac{3}{4}-3\tfrac{1}{2}=\frac{11}{4}-\frac{7}{2}=\frac{11-14}{4}=\frac{-3}{4}$

If 125 x centimetre per second = (x+7) km/hour then the value of x is

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 4

  2. 3

  3. 2

  4. 1

Show answer
Correct Option: C
Explanation:

Given 125 x cm \second=(x+7)x km\hr 

WE know that 125 cm \ sec =4.5 km \ hr
$\therefore 125x cm\ sec =4.5 x km/hr$
Then $4.5 x=(x+7)\Rightarrow 4.5x-x=7\Rightarrow 3.5 x=7 \Rightarrow x=2$

If 5 labourers can make 5 mats in 5 hours then the number of mats that 10 labourers can make in 10 hours are

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 10

  2. 20

  3. 15

  4. 100

Show answer
Correct Option: B
Explanation:

Given 5 mates in 5 hours makes 5 laborers

Then 10  laborers can make in 10 hours =$\frac{5\times 10\times 10}{5\times 5}=20$

The average weight of 7 men is diminished by 3 kg when one of them who weighs 73 kg is replaced by a new man then the weight of the new man is 

maths operations adding and subtracting numbers using place value addition & subtraction mental additions and subtractions

  1. 50 kg

  2. 52 kg

  3. 53 kg

  4. 54 kg

Show answer
Correct Option: B
Explanation:

The averages weight of seven man diminished by 3 kg .Then total weight diminished by $3\times 7=21 kg $

If one of them whose weight is 73 kg is replased by anew man 
Then weight of new man =72-21=52 kg