Question

marcia has $220,000 saved for her retirement. How long will it take for the investment to double in value if it earns 6% interest compounded continuously?

Answer 1

See http://www.moneychimp.com/articles/finworks/continuous_compounding.htm
for the exact answer (11.56 yrs), or use the "rule of 72*" and get about 12 years.

*The product of the annual interest rate and the number of years must be 72 to double your money.

Answer 2

i didn't get the right answer :[

the answer must be 139 months

Answer 3

In your case,
440000=220000*e^0.06t
2=e^0.06t

ln2 = 0.06t
t= ln2/0.06
= 11.5524 years x 12 months
=139 months