10th
posted by ramana .
what rate willl an amount double itself in 20 years at simple interest?

(1.00+x)^20 = 2 where x is the percent. If we call 1.00 + x = Y, then
(Y)^20 = 2
20*log(Y) = log 2
20*log(Y) = 0.30103
log Y = 0.30103/2 =
Solve for y and that will tell you what 1+interest rate must be. Subtracting 1.00 will give you the interest rate. I get something like 3.5% 
In 20 years you must make one times the original amount in interest.
20 x = 1
x = .05 or 5% 
It said simple, not compound.

oops. You're right. I did compound.