Question

at what rate per cent will a sum of rs 4000 yield as rs 1324 as compound interest

Answer 1

let the percentage be r%

amount = 4000+ 1324 = 5324

now we know

P[1+(r/100)]^3 = Amount

4000[1+(r/100)] = 5324
4000 x [(100 + r )/ 100]^3 = 5324
[(100 + r )/ 100]^3 = 5324/4000
(100 + r /100)^3 = 1331/1000
100 + r /100 = 11/10
100 + r = 110
r = 110-100
r = 10

rate % = 10%

Answer 2

Given,

Compound Interest C.I. = Rs. 1324

Principal P = Rs. 4000

Amount A = Compound Interest + Principal

= 1324 + 4000

= Rs. 5324

Let the time t be 3 years (∵ it is not given) and rate percent or rate of interest be r%.

A = P(1 + r/100)³

5324 = 4000(1 + r/100)³

5324/4000 = (1 + r/100)³

1331/1000 = (1 + r/100)³

(11/10)³ = (1 + r/100)³

11/10 = 1 + r/100

(11 - 1)/10 = r/100

(11 - 10)/10 = r/100

1/10 = r/100

1 * 100 = 10 * r

100 = 10r

r = 100/10

r = 10%

Rate of interest = 10% Ans.