Detailed Notes: Averages | Quantitative for GMAT PDF Download

Introduction

• This chapter forms the backbone concept of most questions in the Quantitative Aptitude & Data Interpretation sections. This is a crucial chapter & quick solving methods in this concept will help you save time - which is an essential factor for your success.
  

What is an Average

• Average (or Mean) is defined as the ratio of the sum of the quantities to the number of quantities.
• Following are some examples on average.

Example 1: Find the average of first five natural numbers. 

• Sol: First five natural numbers:

  1, 2, 3, 4, 5

  Step 1: Calculate the sum

  Sum = 1 + 2 + 3 + 4 + 5

  Sum = 1 + 2 + 3 + 4 + 5= 15

  Step 2: Count the total number of values

  Total numbers = 5

  Step 3: Calculate the average

  Average = 155 = 3

  Therefore, The average of the first five natural numbers is 3.

Example 2: If a person with age 45 joins a group of 5 persons with an average age of 39 years. What will be the new average age of the group?

• Sol: Total age will be 45 + 5× 39 = 240. And there will be 6 persons now.
  So, the average will be 240/6 = 40.
  (or)
  Since, 45 is 6 more than 39, by joining the new person, the total will increase by 6 and so the average will increase by 1.
  So, the average is 39 + 1 = 40.

Example 3: Two students with marks 50 and 54 leave class VIII A and move to class VIII B. As a result, the average marks of class VIII A fall from 48 to 46. How many students were there initially in class VIII A?

• Sol: The average of all the students of class VIII A is 46, excluding these two students.
  They have 4 and 8 marks more than 46. So, with the addition of these two students, 12 marks are adding more, and hence the average is increasing 2.
  There should be 6 students in that class, including these two. This is the initial number of students.

Weighted Mean

• Let N₁, N₂, N₃, …. Nn be the weights attached to variable values X₁, X₂, X₃, …….. X_n respectively. Then the weighted arithmetic mean, usually denoted by:
  
• For any two different quantities taken in different ratios. The weighted average is just like a see-saw. 
  The more the ratio of a quantity,  more will be the inclination of the average from mid-value towards the value with more ratios.

Example: The average marks of 30 students in a section of class X are 20 while that of 20 students of the second section is 30. Find the average marks for the entire class X.

Sol: Given:

• Section A: Number of students = 30, Average marks = 20
• Section B: Number of students = 20, Average marks = 30

Step 1: Calculate Total Marks for Each Section

Total marks for Section A:

Total Marks_A = 301 × 20 = 600

Total marks for Section B:

Total Marks_B = 201 × 30 = 600

Step 2: Calculate Total Marks for the Entire Class

Total Marks_class = Total Marks_A + Total Marks_B = 600 + 600 = 1200

Step 3: Calculate Total Number of Students

Total Students = 30 + 20 = 50

Step 4: Calculate the Average Marks for the Entire Class

Average Marks_class = 120050 = 24

Therefore , average marks of the entire class is 24.

Real Facts About Average

Fact 1: If each number is increased/decreased by a certain quantity n, then the mean also increases or decreases by the same quantity.

Fact 2: If each number is multiplied/ divided by a certain quantity n, then the mean also gets multiplied or divided by the same quantity.

Fact 3: If the same value is added to half of the quantities and the same value is subtracted from the other half quantities, then there will not be any change in the final value of the average.

Average Speed

• It is the total distance traveled divided by the time it takes to travel the distance.

• If d₁ & d₂ are the distances covered at speeds v₁ & v₂ respectively and the time taken are t₁ & t₂ respectively, then the average speed over the entire distance (x₁ + x₂) is given by:
  

• If both the distances are equal i.e. d₁ = d₂ = d then,   (i.e. Harmonic mean of two velocities)
• But if both the time taken are equal i.e. t₁ = t₂ = t then,
  Average speed =  (i.e. Algebraic mean of two velocities)

Example: The average of 10 consecutive numbers starting from 21 is:

Sol: The average is simply the middle number, which is the average of 5^th & 6^th no. i.e, 25 & 26 i.e. 25.5

Examples

Example 1: A man travels 120 km at 60 km per hour and returns the same distance at 40 km per hour. What is his average speed for the entire journey?

• Sol: The formula for average speed in a round trip is:
• 2 × Speed₁ × Speed₂Speed₁ + Speed₂
• Here, Speed₁ = 60 km per hour and Speed₂ = 40 km per hour.
• Using the formula:
• 2 × 60 × 4060 + 40 = 4800100 = 48 km per hour
• Therefore , the answer to this question is 48 km per hour

Example 2: A grocer mixes two types of sugar. Type A costs ₹30 per kg, and Type B costs ₹50 per kg. How many kilograms of Type B sugar must he mix with 10 kg of Type A to get a mixture worth ₹40 per kg?

• Sol: Let x be the quantity of Type B sugar in kg.
• Using the weighted average formula:
• 30 × 10 + 50 × x10 + x = 40
• Now, simplify the equation:
• 300 + 50x = 40 × (10 + x)
• 300 + 50x = 400 + 40x
• Rearrange terms:
• 50x – 40x = 400 – 300
• 10x = 100
• x = 10

Example 3: A company has three departments:

1. Department A has 10 employees with an average salary of ₹60,000.
2. Department B has 15 employees with an average salary of ₹80,000.
3. Department C has 5 employees with an average salary of ₹50,000.
   What is the overall average salary of all employees in the company?

Sol: First, calculate the total salary for each department:

• Department A: 10 × 60,000 = 600,000
• Department B: 15 × 80,000 = 1,200,000
• Department C: 5 × 50,000 = 250,000

Now, sum all the salaries and divide by the total number of employees:

600,000 + 1,200,000 + 250,00010 + 15 + 5 = 2,050,00030 = 68,333.33

Answer: ₹68,333.33

Example 4: A jar contains 10 liters of a solution with 30% salt. Two liters are removed and replaced with pure water. What is the new concentration of salt?

• Sol: Step 1: Initial amount of salt in the solution =  30 × 10100 = 3 liters of salt
• Step 2: Salt removed = 30 × 2100 = 0.6 liters of salt
• Step 3: Remaining salt after removal: 3 − 0.6 = 2.4 liters.
• The total volume of the solution is still 10 liters after replacing the removed 2 liters with pure water.
• Step 4: New concentration of salt =  2.410 = 0.24 or 24%

Example 5: The average marks of 3 classes are as follows:

• Class A: 50 students with an average of 75.
• Class B: 30 students with an average of 80.
• Class C: 20 students with an average of 85.
  What is the overall average of all the students?

Sol: First, calculate the total marks for each class:

• Class A: 50 × 75 = 3,750
• Class B: 30 × 80 = 2,400
• Class C: 20 × 85 = 1,700

Now, sum the total marks and the total number of students:

Total marks:

3,750 + 2,400 + 1,700 = 7,850

Total number of students:

50 + 30 + 20 = 100

The overall average is:

7,850100 = 78.5