Lesson Problems on continuously compounded accounts

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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  %281%2B0.08%2F2%29%5E2 = 1.04%5E2 = 1.0816.


Option 2 has one year growing coefficient e%5E0.0775 = 2.71828%5E0.0775 = 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 = 2000%2Ae%5E%280.05%2At%29.


Divide both sides by 2000

    4500%2F2000 = e%5E%280.05%2At%29.


It is the same as

    2.25 = e%5E%280.05%2At%29.


Take natural logarithm of both sides

    ln(2.25) = 0.05*t,

    t = ln%282.25%29%2F0.05 = 16.22.


Round to one decimal place

    t = 16.2 years.    ANSWER


My other lessons on Finance problems in this site are
    - Problems on simple interest accounts
    - Problems on discretely compounded accounts
    - Find future value of an Ordinary Annuity
    - Find regular deposits for an Ordinary Annuity
    - How long will it take for an ordinary annuity to get an assigned value?
    - Find future value for an Annuity Due saving plan
    - Regular withdrawals from an annuity account
    - Ordinary annuity account with non-zero initial deposit as a combined total of two accounts
    - Annual depositing and semi-annual compounding in ordinary annuity saving plan
    - Variable withdrawals from a compounded account (sinking fund)
    - Present value of an ordinary annuity cumulative saving plan
    - Problems on sinking funds
    - Find the compounding rate of an ordinary annuity
    - Accumulate money using ordinary annuity; then spend money via sinking fund
    - Calculating a retirement plan
    - Accumulating money via ordinary annuity and spending simultaneously via sinking fund
    - Loan problems
    - Mortgage problems
    - Amortizing a debt on a credit card
    - One level more complicated non-standard problems on ordinary annuity plans
    - One level more complicated problems on sinking funds
    - One level more complicated non-standard problems on loans
    - Using Excel to find the principal part of a certain loan payment
    - Using Excel to find the interest part of a certain loan payment
    - Tricky problems on present values of annuities
    - OVERVIEW of my lessons on Finance section in this site

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