maths
posted by indu .
an amount gets doubled in 15 years with compund intrest.In how many years amount will become 8 times,using the same compund intrest ?

maths 
tchrwill
an amount gets doubled in 15 years with compund intrest.In how many years amount will become 8 times,using the same compund intrest ?
S = P(1 + i)^n where P = the invested amount, i = the decimal periodic interest rate and n = the number of inrterest paying periods.
Therefore, 2 = 1(1 + i)15 (assuming interest compounded yearly)
Solving, i = .04729 making the annual interest rate of 4.729%.
At the same interest rate and AS/P = 8,
8 = (1.047829)^n.
solving, n = ~45 years.
Checking, S = 1(1.04729)^45 = ~8.
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