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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

Hi,

A bank account starts out with $10,000. Interest is compounded monthly at 6 percent per year (0.5 percent per month)

what is compounded monthly at 6 percent per year!

it's compounded monthly! why he wrote next 6% per year!

what is the meaning of that

is that mean the 10,000 will be 10500 than after a year it will be $10500 * 12months = $126000

after that I will add the 6% to the $126000??? = $133560

I don't think I'm right lol I'm just saying hehe

can u help it's very important I felt nervous because I didn't understand that

can you please analyse the question for me

Wisdom is a tree which grows in the heart and fruits on the tongue

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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

and the full exercise is :

Write an algorithm to settle the following question: A bank account starts out with $10,000. Interest is compounded monthly at 6 percent per year (0.5 percent per month). Every month, $500 is withdrawn to meet college expenses. After how many years is the account depleted?

the problem is I don't want only the answer I want to analyse the answer and the steps to have fully understand about that problem

Wisdom is a tree which grows in the heart and fruits on the tongue

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,596

hi Hannibal lecter

So that savers can compare schemes it is usual to state the yearly (annual) rate of interest. Some schemes then calculate and add interest once per year. But some schemes add interest every month, so it is necessary to state what rate will be applied for this. I live in the UK and here there is a formula that does this. It's more complicated than what is given in your question so I won't confuse you by describing it now. The USA system for working out a monthly rate is to divide the yearly rate by 12. Thus 6% per year becomes 0.5% per month.

Each month this is what happens:

Start with the amount in the account.

Add interest from the previous period.

Subtract $500.

This gives the new amount that will apply for the next interest calculation, one month later.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Sometimes I deliberately make mistakes, just to test you! …………….Bob Bundy

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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

oh I didn't notice that how could I didn't focus I was focus in read this but u helped me finally

I have another question

the account won't be depleted because when we add 500 to 10,000

than sub 500

nothing will change the account won't be $0

what is the value in the question that I can change to make the account depleted ?

Wisdom is a tree which grows in the heart and fruits on the tongue

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,596

The interest after the first month is $50 not $500.

10000 x 0.5/100 = 50

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Sometimes I deliberately make mistakes, just to test you! …………….Bob Bundy

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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

but please look at this this :-

Consider the question in Exercise R1.12. Suppose the numbers ($10,000, 6 percent, $500) were user selectable. Are there values for which the algorithm you developed would not terminate? If so, change the algorithm to make sure it always terminates

it was the next question after that

what the question want from me to do?

Wisdom is a tree which grows in the heart and fruits on the tongue

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,596

Let's try an algebraic approach.

Let the initial amount in the account be P; the monthly interest rate be r; and the amount taken out each month be M.

At the end of each month

new amount in the account = old amount plus interest less the monthly amount out

P_new = P_old + (P_old x r/100) - M

If P_new is to be the same as P_old then

P = P + P x r/100 - M

So M = P x r/100

If I choose P = 10000 and r = 0.5 then

M = 10000 x 0.5/100 = 100 x 0.5 = 50

So the algorithm will not end if M = 50

You could vary a different value.

If r = 0.5 and M = 500 then

500 = P x 0.5/100 = P/200 so P = 100,000

So if you have a much bigger amount saved then the interest earned can exactly cover the monthly amount out.

In the above I have kept the P constant from month to month. You could also let the amount in the account grow. So P = 10000, r = 0.5, M = 40 means that P will grow, month by month.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Sometimes I deliberately make mistakes, just to test you! …………….Bob Bundy

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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

you said :

new amount in the account = old amount plus interest less the monthly amount out

you mean : p_new = 10000 + 10050 - 500 = 19550 ?

but I thought the amount will be only : 10000 + 0.5% = 10050 in the first month

you are saying old amount plus interest

why did you put two p_old!

sorry but I didn't understand the first movements

it's like you rote p_new = p_old + M - M !

because M = ( p * r/100 )

why this step?

can you give me values step by step for the first 3 months please

before that pls see how is my calculations :-

balance = 10,000 + 0.5% = 10050

now 10050 - $500 = 9950

now add 9950 + 0.5% = 9597.75

after that again sub 9597.75 - $500

to the end....

and in my calculations the algorithm will stop because the account balance will be less than $500 so can't withdraw from it again

what is wrong in my calculations?

*Last edited by Hannibal lecter (2019-09-13 15:57:04)*

Wisdom is a tree which grows in the heart and fruits on the tongue

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,596

hi

I used Microsoft Excel to do a few years' calculations. I have made an image of these and alongside the formulas I used to do the calculations.

You should be able to see that the 'Principle' is gradually going down but not as fast as you thought.

Hope that answers all your questions.

Bob

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Sometimes I deliberately make mistakes, just to test you! …………….Bob Bundy

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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 255

thank you very much

Wisdom is a tree which grows in the heart and fruits on the tongue

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