First slide

Arithmetic progression

Question

A man saves Rs. 200 in each of the first 3 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.

Moderate

A

21 months
B

18 months
C

19 months
D

20 months

Solution

Total saving = 200+200+200+240+280+… to n months = 11040
$\begin{aligned} \Rightarrow 400 + \frac{n - 2}{2} [400 + (n - 3) 40] = 11040 \\ \Rightarrow (n - 21) (n + 26) = 0 \Rightarrow n = 21 \end{aligned}$

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