### The average of some natural numbers is 15. If 30 is added to the first number and 5 is subtracted from the last number the average becomes 17.5 then the number of natural numbers is –

A. 20 B. 30 C. 15 D. 10 Answer: Option D

### Solution(By Apex Team)

Let the number of natural numbers = n ∵ The average of some natural numbers = 15 ⇒ Sum of these natural number = 15 × n = 15n ∵ 30 is added and 5 is subtracted So, now addition of these number = 15n + 30 – 5 = 15n + 25 According to the question, $\begin{array}{l}\Rightarrow\left(\Large\frac{15n+25}{n}\right)=17.5\\ \Rightarrow15\mathrm{n}+25=17.5\mathrm{n}\\ \Rightarrow2.5\mathrm{n}=25\\ \Rightarrow\mathrm{n}=10\end{array}$ Therefore, The numbers of natural numbers n = 10

## Related Questions On Average

A. 20
B. 21
C. 28
D. 32

A. 18
B. 20
C. 24
D. 30

A. 10 years
B. 10.5 years
C. 11 years
D. 12 years

### If the arithmetic mean of 0, 5, 4, 3 is a, that of -1, 0, 1, 5, 4, 3 is b and that of 5, 4, 3 is c, then the relation between a, b, and c is.

A. a = b = c
B. a : b : c = 3 : 2 : 4
C. 4a = 5b = c
D. a + b + c = 12