I $720 P=$1000 r=9

Answer 1

A = P(1+r/n)^(nt) where P=Principal, r= annual interest Rage, I = total Interest earned in t years

r=9% = .09

P=1000

I = 720

let n= 1 for annual compounding

A=Amount in t years at r rate of interest compounded n times per year = P+I = 1,720

A = 1000(1.08)^t

1720/1000 = 1.08^t

1.72 = 1.08^t

log_1.081.72 = t

t = ln1.72/ln1.08

= about 7.407 years to get $720 interest

or if it were compounded continuously

then

1.72 = e^rt = e^.08t

.08t = ln1.72

t = (ln1.72)/.08 = about 6.779 years

for compounding between annually and continuously

6.8 < t < 7.4

or at simple interest with no compounding

then 720 = 1000(rt) = 80t

t = 720/80 = 72/8 = 9 years

that comes out so evenly, that just might be the rest of the problem you left out. Put in "simple" and it's far simpler to solve