Question

$ans plzz$
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Answer 1

A.P. = 320, 360, 400...

a = 320

d = 360 - 320 = 40

Sn = 20000

Formula =>

Sn = n/2(2a + (n -1)d)

20000 = n/2(2×320 + (n - 1)40)

20000 = n(320 + (n - 1)20)

20000 = n(320 + 20n - 20)

20000 = 300n + 20n²

20n² + 300n - 20000 = 0

20(n² + 15n - 1000) = 0

n² + 15n - 1000 = 0

n² + 40n - 25n - 1000 = 0

n(n + 40) - 25(n + 40) = 0

(n + 40)(n - 25) = 0

n = -40 or 25

Months can't be negative so it should be positive.

Therefore in 25 months it will save ₹20000