Permutation and CombinationHard

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

Options

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

Solution


1             2             3          4           5           6       .....
200       200       200       240       280       .....       .....
Sum = 11040
120 + 80 + 160 + 40 + 200 + 240 + .... = 11040
[2a + (n - 1)d] + 80 + 40 = 11040
[2a + (n - 1)40] = 10920
n [ 6 + n - 1 ] = 546
n (n + 5) = 546
n = 21

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