At 6.5 percent interest, how long does it take to double your money? To quadruple it?

Answer:

We use the formula:
A=P(1+r/100)^n
where
A=future value
P=present value
r=rate of interest
n=time period.

A= P ( 1+ 6.5/100) ^ n

Let P = $ x

Hence, A= $ 2x

Hence, 2x = x*(1+6.5/100) ^n

or 2 = (1.065) ^ n

Taking log on both sides we get,

log 2 = n log (1.065)

Hence, n = Log 2 / log (1.065)

or n = 11.00673904

Hence the correct answer is 11 years

We use the formula:
A=P(1+r/100)^n
where
A=future value
P=present value
r=rate of interest
n=time period.

A= P ( 1+ 6.5/100) ^ n

Let P = $ x

Hence, A= $ 4x

Hence, 4x = x*(1+6.5/100) ^n

or 4 = (1.065) ^ n

Taking log on both sides we get,

log 4 = n log (1.065)

Hence, n = Log 4 / log (1.065)

or n = 22.01347808

Hence the correct answer is 22.01 years