Question

The average age of boys in a class of 30
is 15 years. If 10 more boys join the
class, the average age of the whole class
is reduced by one year. What is the
average age of new comers?​

Answer 1

Answer:

The formula to calculate average of a distribution is given as:

$\boxed{\textbf{Average} = \dfrac{\textbf{Sum of all observations}}{\textbf{Total no. of observations}}}$

According to the question,

Average age of 30 boys in a class is 15 years. Writing it in terms of the above formula we get:

$\implies 15 = \dfrac{\text{Sum of all observations}}{30}\\\\\\\implies \text{Sum of all observations} = 30 \times 15\\\\\implies \boxed{\text{Sum of all observations} = 450}$

Now, it is given that 10 more boys join the class. Hence total number of students increase from 30 to 40.

It is also given that Average age becomes 14 years. Hence we need to find the average age of newcomers.

Let the sum of ages of the new boys be 'x'.

Therefore the sum of all observations after newcomers enter is given as:

⇒ Sum = 450 + x

Substituting in the Average formula we get:

$\implies 14 = \dfrac{450+x}{40}\\\\\\\text{Cross multiplying we get:}\\\\\implies 450 + x = 14 \times 40\\\\\implies 450 + x = 560\\\\\implies x = 560 - 450\\\\\implies \boxed{x = 110}$

Hence the sum of ages of newcomers is 110.

Finding the average age of newcomers we get:

$\implies \text{Average} = \dfrac{110}{10}\\\\\implies \boxed{ \bf{\textbf{Average} = 11\:years}}$

Hence 11 years is the required answer.

Answer 2

Given information:

• Average age of boys of 30 students = 15 years
• 10 more boys joined the class.
• Average age for reduced by 1 year

Need to find?

• Average age of new corners

Formula needed:

• Average = Sum of observations / Total number of observations.

‎

What to assume?

• So let us take sum of observations ages be y.

‎

Step by step explaination:

Let us divide the question into two parts such as part 1 and part 2.

Part 1.

First of all we needed to find out the sum of all observations. That can too be calculated by the formula of average.

Let's use that formula.

Average = Sum of observations / Total number of observations.

Substituting values.

→ 15 = Sum of observations / 30

Reversing sides.

→ Sum of observations = 30×15

Now multiplying.

→ Sum of observations = 450

Part2.

According to the question 10 more boys joins the class:

→ 30 + 10

→ 40

Sum of observations:

→ 450 + y

Substituting the values again in the given formula.

Let's do it quickly!!

→ Average = Sum of observations / Total number of observations.

We will take average as 14 because average age of whole class is reduced by 1 year.

→ 14 = 450+y / 40

→ 450+y = 14×40

→ 450+y = 560

Reversing sides.

→ y = 560-450

→ y = 110

Finding average..

Average = Sum of observations / Total number of observations.

Average = 110/10

Reducing to their lowest terms.

Average = 11

Answer:

• Average age of new corners is 11 years.