what formula will i use for this word problem?

Question

How many yrs will it take 1000 dollars invested at 7% interest to earn 280 dollars?

what formula will i use for this?

Answer 1

Simple or compound interest?

I'll guess at Simple since you didn't state which.

A = Prt

Where A is the money earned
P = what you invested
r = rate
t = years

280 = 1000 * (0.07) * t
280 = 70 t
4 = t

Answer 2

The formula for interest
I = N x P x R / 100 where N is the number of years, P is the Principal amount and R is the Rate of Interest for an year.

So, we can write

280 = N x 1000 x 7 / 100

N = 280 x 100 / 7000 = 4

It takes 4 years. Pl. note that this is valid if the Rate of Interest is Simple. If Compound Interest is involved, we need to know the frequency of compounding, monthly, quarterly, half-yearly or yearly. If T is the time period for compounding the interest, we have

I = P x (1 + R/100)^N.T where R is now the rate of interest for the period T and N.T represents the number of Time periods.

Answer 3

If i is the interest, p the principal, r the annual percentage rate, t the time in years, these are the formulae.

For simple interest:
i = prt / 100

For compound intererest:
i = p[ (1 + r / 100)^t - 1].

Answer 4

i dont know about formulas, nvm