model 4 find nth average from 1st & last number Section-Wise Topic Notes With Detailed Explanation And Example Questions

Answer: (c)

Using Rule 1,

Average of two or more numbers/quantities is called the mean of these numbers, which is given by

$\text"Average(A)" = \text"Sumof observation / quantities"/\text"No of observation / quantities"$

∴ S = A × n

According to question,

The average of 9 numbers = 30

Now,The total of 9 numbers

= 30 × 9 = 270

The average of first 5 numbers = 25

∴ The total of first 5 numbers = 25 × 5 = 125

and Total of last 3 numbers = 3 × 35 = 105

Hence, required answer

= 270 – (125 + 105)

= 270 – 230 = 40

Answer: (c)

Using Rule 1,

Numbers in order

⇒ a, b and c

∴ a + b = 2 × 2 = 4

b + c = 2 × 3 = 6

c + a = 2 × 4 = 8

On adding,

2 (a + b + c) = 4 + 6 + 8 = 18

⇒ a + b + c = $18/2$ = 9

∴ Required average =$9/3$ = 3

Answer: (a)

Using Rule 1,

x + x + 1 + x + 2 + x + 3 + x + 4 + x + 5 + x + 6 + x + 7 = 6.5 × 8 = 52

⇒ 8x + 28 = 52

⇒ 8x = 52 – 28 = 24

⇒ x = 3

∴ Required average =${3+10}/2$= 6.5

Answer: (b)

Using Rule 1,

Average of two or more numbers/quantities is called the mean of these numbers, which is given by

$\text"Average(A)" = \text"Sumof observation / quantities"/\text"No of observation / quantities"$

∴ S = A × n

Last number

= 30 × 12 – 20 × 11 – 9 × 10

= 360 – 220 – 90

= 360 – 310 = 50

Answer: (b)

Using Rule 1,

Average of two or more numbers/quantities is called the mean of these numbers, which is given by

$\text"Average(A)" = \text"Sumof observation / quantities"/\text"No of observation / quantities"$

∴ S = A × n

a + b + c = 18 × 3 = 54

and, b + c + d = 16 × 3 = 48

∴ a + b + c – b – c – d

⇒ 54 – 48 = 6

⇒ a – d = 6

⇒ a – 19 = 6

⇒ a = 19 + 6 = 25

Answer: (b)

Using Rule 1,

Average of two or more numbers/quantities is called the mean of these numbers, which is given by

$\text"Average(A)" = \text"Sumof observation / quantities"/\text"No of observation / quantities"$

∴ S = A × n

Fourth number

= (4×4+4×4 – 3×7)

= (16 +16-21) = 11