Question

Let $N$ be the set of natural numbers. Suppose $f:N\to N$ is a function satisfying the following conditions :
$\left(a\right)f(m+n)=f\left(m\right)+f\left(n\right)\left(b\right)f\left(2\right)=2$
Then the value of $\frac{1}{7}\sum _{k=1}^{20}f\left(k\right)$ is

• A. $35$
• B. $40$
• C. $30$
• D. $20$

Answer 1

The correct option is C $30$

$f(m+n)=f\left(m\right)+f\left(n\right)$
Putting $m=n=1,$ we get
$f\left(2\right)=2f\left(1\right)=2\Rightarrow f\left(1\right)=1$
Putting $m=2,n=1,$ we get
$f\left(3\right)=f\left(2\right)+f\left(1\right)=3$
Putting $m=n=2,$ we get
$f\left(4\right)=2f\left(2\right)=4$
Similarly, $f\left(n\right)=n$

Therefore, $f\left(x\right)=x$ for all $x\in N$
Now, $\frac{1}{7}\sum _{k=1}^{20}f\left(k\right)$
$=\frac{1}{7}\sum _{k=1}^{20}k=\frac{20\times 21}{7\times 2}=30$