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Problems on continuously compounded accounts
Problem 1
Parents of a newborn baby are given a gift of Php 50,000 and will choose between two options to invest
for their child’s college fund. Option 1 is to invest the gift in a fund that pays an average annual
interest rate of 8% compounded semiannually; option 2 is to invest the gift in a fund that pays
an average annual interest rate of 7.75% compounded continuously. Which is the better option, assuming
each investment has a term of 18 years?
Solution
Let's calculate one year coefficients of growth: it is enough to make a selection/conclusion.
Option 1 has one year growing coefficient = = 1.0816.
Option 2 has one year growing coefficient = = 1.08058 (rounded).
Comparing, it is clear that option 1 is better (without making long calculations for 18 years).
Problem 2
Mia invests $2,000 in a money market account that earns 5% annual interest compounded continuously.
Approximately how many years will it take her money to grow to the $4,500 she needs for her small business start-up?
Solution
Using the formula for continuously compounded account, write
4500 = .
Divide both sides by 2000
= .
It is the same as
2.25 = .
Take natural logarithm of both sides
ln(2.25) = 0.05*t,
t = = 16.22.
Round to one decimal place
t = 16.2 years. ANSWER