Question

A man saves Rs 200 in each of the first three months of his service. In each of the subsequent months his saving increases by Rs 40 more than the saving of immediately previous month. His total saving from the start of service will be Rs 11040 after

• A. 19 months
• B. 20 months
• C. 21 months
• D. 18 months

Answer 1

The correct option is C 21 months

Let required number of months = n
$\therefore 200\times 3+(240+280+320+...+(n-3{)}^{th}term)=11040\Rightarrow \frac{n-3}{2}[2\times 240+(n-4)\times 40]=11040-600\Rightarrow (n-3\left)\right[240+20n-80]=10440\Rightarrow (n-3\left)\right(20n+160)=10440\Rightarrow (n-3\left)\right(n+8)=522\Rightarrow n^2+5n-546=0=\Rightarrow (n+26\left)\right(n-21)=0\therefore n=21$