Sponge bob Annuties word problems

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Hey I don't get annuity porblems..I don't get how you know what equation you have to use and stuff. like for these questions:

To save money for a new pinapple, Spongebob plans to deposit @250 into an account at the end of every three months for the next two years. He will begin making payments three months from now. If his count has an interest rate of 4% per annum, compounded quarterly, how much will sponge bob have after he makes his last payment??

[I don't get 'math english' -_-']

Squidward wants to invest $20 000. He must decide between a 12 year plan with an interest rate at 8% per annum, compounded quarterly,and a 12 year plan with an interest rate of 7.75% per annum, compounded monthly. Which plan earsn Squidward more interest, and by how much?
 
anna, at an annual rate of 9.5%, how much interest
will be earned on $4200 for a year?

...just to see where you're at...
 
umm I think it's this formual.. A=p(1+i)^n
A=?
p=$4200
i= .095/1
n=1

A=4200(1+.095)^1
=4599

I'm not sure..I don't get the difference between questions that ask you the sum formula: Sn= a(r^n-1)/r-1 and the other formual which is:
A=p(1+i)^n

I need major help on this..I got a test monday =(
 
anna said:
To save money for a new pinapple, Spongebob plans to deposit @250 into an account at the end of every three months for the next two years. He will begin making payments three months from now. If his count has an interest rate of 4% per annum, compounded quarterly, how much will sponge bob have after he makes his last payment??

This is what the account would look like if no interest was paid:
3: 250
6: 500
9: 750
12: 1000
15: 1250
18: 1500
21: 1750
24: 2000
You ok with that?

However, some interest will be paid, so account will end up > 2000.
Interest will be 1% per quarter (3 month period): still with me?
The formula is:
A = p[(1 + i)^n - 1] / i
A = Amount at end : ?
p = payment : 250
i = interest per quarter: .04/4 = .01
n = number of quarters: 2 * 4 = 8

SO: A = 250[(1.01)^8 - 1] / .01 = 2071.42~

Squidward wants to invest $20 000. He must decide between a 12 year plan with an interest rate at 8% per annum, compounded quarterly,and a 12 year plan with an interest rate of 7.75% per annum, compounded monthly. Which plan earsn Squidward more interest, and by how much?

Simply use the formula A = p(1 + i)^n : you seem to know this one

A = 20000(1 + .08/4)^48

A = 20000(1 + .0775/12)^144

Which one is the best?
Click to expand...
 
Ok I get it now, except for the spongebob one, we're not suppose to use A = p[(1 + i)^n - 1] / i but Sn=A(r^n-1)/r-1
so it would be, "To save money for a new pinapple, Spongebob plans to deposit @250 into an account at the end of every three months for the next two years. He will begin making payments three months from now. If his count has an interest rate of 4% per annum, compounded quarterly, how much will sponge bob have after he makes his last payment?? "

Sn=A (r^n-1)/r-1
Sn= 250/1+.04/4[( 1.04/4^8) -1 ]/ (1.04/4 - 1)
I think that equation is right...It's hard to do these question on the computer with all the / and ^ It's easier if I could draw it out...
 
anna said:
Ok I get it now, except for the spongebob one, we're not suppose to use A = p[(1 + i)^n - 1] / i but Sn=A(r^n-1)/r-1
so it would be, "To save money for a new pinapple, Spongebob plans to deposit @250 into an account at the end of every three months for the next two years. He will begin making payments three months from now. If his count has an interest rate of 4% per annum, compounded quarterly, how much will sponge bob have after he makes his last payment?? "

Sn=A (r^n-1)/r-1
Sn= 250/1+.04/4[( 1.04/4^8) -1 ]/ (1.04/4 - 1)
I think that equation is right...It's hard to do these question on the computer with all the / and ^ It's easier if I could draw it out...
Click to expand...

The formula you're "suppose" to use: Sn=A (r^n-1)/r-1
is the SAME as the one I'm using: the "r" in yours = 1 +.04/4 = 1.01;
the (1 + i) in mine = 1 + .04/4 = 1.01.
Anyway, whoever is asking to use that formula of yours is being
ridiculous, as it is NOT the standard formula, plus complicates things
with the "r": who in **** "knows" that r represents 1+i ?
 
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