ICSE Class 10 Answered
Preeti has a recurring deposit account of Rs 1000 per month at 10%per annum. If she gets Rs 5550 as interest at the time of maturity, fund the total time for which the account was held.
Formula is this:SI =Px1/2n(n+1)x 1/12xR /100
Recurring deposit interest I is given by,
I = P×[ n(n+1)/(2×12) ] (r/100) .............. (1)
where P is monthly contribution, P = Rs. 1000
n = number of months,
r = rate of interest = 10%
and I = interest = Rs.5550
By substituting the values in eqn.(1), we get
1000×[ n(n+1)/24 ]×(10/100) = 5550 or n(n+1) = (24/100)×5550 = 1332
Therefore, n2 + n - 1332 = (n+37) (n-36) = 0
n = -37 or n= 36
Since, n can't be negative.
Hence, the number of months = 36