Results 1 to 3 of 3

Math Help - compound interest

  1. #1
    Newbie
    Joined
    Apr 2009
    Posts
    10

    Exclamation compound interest

    ok i have two questions that i need help on. ugh word problems...

    1. a car tire has a small leak, and the tire pressure in pounds per square inch after t minutes is given by

    P(t) = 32e (-0.2t) After how many minutes was the pressure 5 pounds per square inch?

    *note the (-0.2t) is the exponent.

    there is not time- so i'm not sure what to plug into (t)? plug in (60) for (t) since its talking about minutes?? and since its neg. i keep getting a neg. answer. but there is no such thing as a neg min. so i'm stuck....

    2. Suppose $8000 is invested into a savings account that accrues intrest continuously at a rate of 2.3% determine how long it would take the investment to triple.

    its continuously...there is no year or quartly so how do i solve?
    i think the formula to use is

    A=P (1 + r/t) ^ nt -----is this right?

    what do i do for t? apparently time that i have the problem with in both questions.


    can anyone help?
    Last edited by b_austintx; April 26th 2009 at 08:30 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Member SengNee's Avatar
    Joined
    Jan 2008
    From
    Pangkor Island, Perak, Malaysia.
    Posts
    155
    Quote Originally Posted by b_austintx View Post
    ok i have two questions that i need help on. ugh word problems...

    1. a car tire has a small leak, and the tire pressure in pounds per square inch after t minutes is given by

    P(t) = 32e (-0.2t) After how many minutes was the pressure 5 pounds per square inch?

    *note the (-0.2t) is the exponent.

    there is not time- so i'm not sure what to plug into (t)? plug in (60) for (t) since its talking about minutes?? and since its neg. i keep getting a neg. answer. but there is no such thing as a neg min. so i'm stuck....

    2. Suppose $8000 is invested into a savings account that accrues intrest continuously at a rate of 2.3% determine how long it would take the investment to triple.

    its continuously...there is no year or quartly so how do i solve?
    i think the formula to use is

    A=P (1 + r/t) ^ nt -----is this right?

    what do i do for t? apparently time that i have the problem with in both questions.


    can anyone help?
    1)
    Given, P(t)=5
    32e^{-0.2t}=5

    2)
    After 1st year, T_1=8000(1.023).
    After 2nd year, T_2=8000(1.023)(1.023).
    After 3rd year, T_3=8000(1.023)(1.023)(1.023).

    It is a G.P.

    a=8000
    r=1.023

    T_1=ar
    T_2=ar^2
    T_3=ar^3

    Given, after n^{th} year, the investment is tripled.

    Therefore, T_n=3a
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Apr 2009
    Posts
    10
    1). i'm still lost. i get that p(t)= 5
    but what about the -0.2t ? do i place the 5 in and multiply?

    which if i did i would get 32e^{-0.2t} =86.98
    ? 86.98 = 5 ?

    what are the steps for this problem?


    2.)

    so after the 3rd year the investment tripled?

    i just take a=8000 and mult. that by 3??

    what do you mean it is a G.P? there wasn't a formula that i needed to use?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Compound interest at changing interest rates
    Posted in the Business Math Forum
    Replies: 2
    Last Post: October 21st 2010, 04:55 AM
  2. compound interest
    Posted in the Business Math Forum
    Replies: 11
    Last Post: March 12th 2010, 08:05 AM
  3. Compound Interest
    Posted in the Business Math Forum
    Replies: 4
    Last Post: November 24th 2009, 06:18 PM
  4. Compound interest
    Posted in the Business Math Forum
    Replies: 1
    Last Post: October 31st 2009, 08:14 PM
  5. Compound Interest
    Posted in the Pre-Calculus Forum
    Replies: 6
    Last Post: October 6th 2009, 04:24 AM

Search Tags


/mathhelpforum @mathhelpforum