Question

the average age of students in a class is 15 years.when the teachers age of 45 years is added. the average age now becomes 18 years. how many student a in the class

Answer 1

Average age of students in a class is 15 years

Let the number of students be x

Sum of ages of all x number of students = 15x

Now when the teachers age of 45 added new average = 18 years

Now Sum of all the ages of x student and teacher = 18(x+1) = 15x+45

i.e 18x + 18 = 15x +45

3x = 27

x = 9

Numbers odf students in class = 9

Answer 2

Answer:

9.

Step-by-step explanation:

Let the number of students in the class be a.

Given, the average age of students in a class is 15 years.

It means :

⇒ ( sum of ages of students ) / ( number of students ) = 15 year

⇒ ( sum of ages ) / a = 15 year

⇒ sum of ages of total students = 15a year           ...( 1 )

According to the question : If the age of teacher is added, new average becomes 18 year. Now, the total number is ( a + 1 ).

⇒ ( sum of ages of students + age of teacher ) / ( a + 1 ) = 18 year

⇒ ( 15a + 45 ) / ( a + 1 ) = 18 year       { from above }

⇒ 15a + 45 = 18( a + 1 )

⇒ 15a + 45 = 18a + 18

⇒ 45 - 18 = 18a - 15a

⇒ 27 = 3a

⇒ 9 = a

Therefor the number of students in the class in 9.