Hi,

If you can invest money at an annual interest rate of r and you want to accumulate $A in t years then the principal you must invest $P is given by

P = A/(1 + rt)

solve for r, the interest rate.

My first thought is to remove the fraction from the expression by multiplying both sides by (1 + rt)

P(1 + rt)= [A/(1 + rt)] (1 + rt)
P(1 + rt) = A

Expand the left side

P + Prt = A

subtract P from each side

Prt = A - P

Finally divide each side by Pt

Prt/(Pt) = (A - P)/(Pt) or
r = (A - P)/(Pt)

Now try your problem.
Penny