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 swrvice will be Rs. 11040 after

Options

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

Answer

C

Explanation

Let the time taken to save Rs. 11040 be `(n+3)` months.
For first 3 months, he saves Rs. 200 each month.
`:. " In " (n+3)" month "`
`3xx200+(n)/(2)[2(240)+(n-1)x40]=11040`
`implies 600+(n)/(2)[40(12+n-1)]=11040`
`implies 600+20n(n+11)=11040`
`implies n^(2)+11n-522=0`
`implies (n-18)(n+29)=0`
`:. n=18, " neglecting " n=-29`
`:. " Total time " =n+3=21 " months "`.