If $1000 is invested in an account paying 12% annually compounded continously, how long will it take to double the investment?

For continuous compounding we use:


so we can write:



taking the natural log of both sides leads to



so time is about 5.78 years

If a person invested a third of her money at 6% and two thirds at 9% and received $450 in interest, find the total money invested.

find out how long it take $2500 investment to double if it is invested at 9% compounded quarterly. Round to the nearest tenth of a year. use the formula A=P(1+r/n)^nt

3000 is invested at 7.5% interested monthly how long does it take to triple round to nearest year?

taking ln of both sides and simplifying leads to about
t=14.7
so say 15 years

How long will it take $2900 being invested at 7% compounded quarterly to double? Use A=P(1+r/n)^nt.

Doubling time does not depend upon amount invested because A=2P and so P divides out of the equation. There is a formula, but the computation is really not that bad:

simplify, take any log of both sides, and...
t=9.99 years (about 10 years)