Quantitative Aptitude

1. The average age of all the students of a class is 18 years. The average age of boys of the class is 20 years and that of the girls is 15 years. If the number
of girls in the class is 20, then find the number of boys in the class. 




Answer & Solution
Answer:

30

Solution:

Let the number of boys in the class be x.

Then, 18 (x + 20) = 20x + 15 × 20

⇒ 18x + 360 = 20x + 300

⇒ 2x = 60

⇒ x = 30.

2. The sum of the three consecutive even numbers is 44 more than the average of these numbers. Which of the following is the third largest of these
numbers? 




Answer & Solution
Answer:

24

Solution:

Let the numbers be x, x + 2 and x + 4.
Then, \((x + x + 2 + x + 4) - {x + x + 2 + x + 4 \over 3} = 44\)
⇒ \((3x + 6) - {3x + 6 \over 3} = 44\)

⇒ 2 × (3x + 6) = 132
⇒ 6x = 120

⇒ x = 20

3. The mean of 25 observations was found to be 78.4. But later on it was found that 96 was misread as 69. The correct mean is 




Answer & Solution
Answer:

79.48

Solution:

​Correct sum = (78.4 × 25 + 96 – 69) = 1987.
Correct mean =\( {1987 \over 25} = 79.48\)

4. Out of three numbers, the first is twice the second and is half of the third. If the average of the three numbers is 56, then difference of first and third
numbers is




Answer & Solution
Answer:

24

Solution:

Let the second number be x. Then, first number = 2x, third number = 4x.
⇒ 2x + x + 4x = 56 × 3

⇒ 7 x = 168

⇒ x = 24

5. The average marks of a student in 4 subjects is 75. If the student obtained 80 marks in the fifth subject, then the new average is




Answer & Solution
Answer:

76

Solution:

Sum of marks in 4 subjects = 75 × 4 = 300
Sum of marks in 5 subjects = 300 + 80 = 380
New average = \({380 \over 5} = 76\)