Question

A man saved Rs. $320$ during the first month Rs. $360$ in the second month Rs. $400$ in the third month, if he continues his saving in this sequence in how many months will be save Rs. $20,000$?

• A. $28$
• B. $25$
• C. $22$
• D. $20$

Answer 1

Answer: (B)

$25$

a=320, d=40
$S_n=20000$
It is required to find n
$S_n=20000=\frac{n}{2}[2\times 320+(n-1)\times 40]$
$\Rightarrow 500=\frac{n}{2}[15+(n-1)]=\frac{n}{2}(n+15)$
$n^2+15n-1000=0$
$\Rightarrow n=25$