Asked by BJ - help please

You invest money that pays 6.3% interest/year compounded quarterly.

How long should you invest if want to double initial investment?

I tried A=P(1+r/n)^nt, but it didn't work, using A=2P. the principle is not given.

Please help

Answered by Steve
That should have worked. The Rule of 72 indicates the investment should double in about 72/6.3 = 11.4 years.

Using the formula,

2P = P(1 + .063/4)^(4t)
2 = (1.01575)^(4t)
ln 2 = 4t ln 1.01575
4t = ln2/ln1.01575 = 44.355
t = 11.08 years

Pretty close to the Rule of 72.
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