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JEE Main Math Class 11 Sequences and Series Part 1 Questions
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© examsnet.com
Question : 76
Total: 100
A man saves
₹
200
in each of the first three months of his service. In each of the subsequent months his saving increases by
₹
40
more than the saving of immediately previous month. His total saving from the start of service will be
₹
11040
after
[2011]
19 months
20 months
21 months
18 months
Validate
Solution:
Let number of months
=
n
∴
200
×
3
+
(
240
+
280
+
320
+
.
.
.
+
(
n
−
3
)
th
term
)
=
11040
⇒
n
−
3
2
[
2
×
240
+
(
n
−
4
)
×
40
]
=
11040
−
600
⇒
(
n
−
3
)
[
240
+
20
n
−
80
]
=
10440
⇒
(
n
−
3
)
(
20
n
+
160
)
=
10440
⇒
(
n
−
3
)
(
n
+
8
)
=
522
⇒
n
2
+
5
n
−
546
=
0
⇒
(
n
+
26
)
(
n
−
21
)
=
0
∴
n
=
21
© examsnet.com
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