Finding the Interest Rate Needed for a Bank Deposit to Double in 8 Years

[Math10]

Homework Statement

At what rate of interest, compounded continuously, will a bank deposit double in value in 8 years?

Homework Equations

None.

The Attempt at a Solution

Q₀e^8r=2Q₀
e^8r=2
8r=ln(2)
r=ln(2)/8
But the answer is (25/2)ln(2)%. How do I get the answer from there?

 

[Ray Vickson]

Homework Statement

At what rate of interest, compounded continuously, will a bank deposit double in value in 8 years?

Homework Equations

None.

The Attempt at a Solution

Q₀e^8r=2Q₀
e^8r=2
8r=ln(2)
r=ln(2)/8
But the answer is (25/2)ln(2)%. How do I get the answer from there?

Be careful about units.

 

[Math10]

Thank you for reminding me about units. I got it! Thanks a lot!

 

[Windadct]

MY father pointed out that doubling of principal occurs - approx 72 / Rate ... while not accurate enough for an answer, a quick check of you are in the ball park... 9 vs 8.66