Question

9) A cumulative deposit of ₹ 2,400 per month at the rate of 10% becomes ₹ 30,100 on maturity. Find the

time period.​

Answer 1

A cumulative deposit of 2400 Rs per month at the rate of 10% becomes 30100 on maturity.

To find : the time period.

solution : if P is the deposit of every month, r is the rate of interest and n is the time period. then interest I of cumulative deposit for this time period is given by,

I = P × n(n + 1)/(2 × 12) × r/100

here P = 2400, r = 10%

here interest, I = A - P = 30100 - 2400n

so, 30100 - 2400n = 2400 × n(n + 1)/24 × 10/100

⇒30100 - 2400n = 10n(n + 1)

⇒30100 - 2400n = 10n² + 10n

⇒10n² + 2410n - 30100 = 0

⇒n² + 241n - 3010 = 0

⇒n = (-241 ± √70121)/2 ≈ 12, -253

Therefore the time period = 1 year

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