figuring out principal from rate, time, interest: Melissa's account earned $14.39...

[steveopolis]

What I can do in this area so far:
*figure out interest due when I know principle, interest rate (percentage), time

I'm facing two questions that ask me to figure out the principal, which seems to indicate all I have to do is the reverse, or divide something. But I can't think of what!

"Melissa's account earned $14.39 in 3 months. Simple interest rate is 2.8%. What was the principal?"

When I know principal, I do:

Principal x [rate as percentage, converted to demical] x time [if 3 months, 1/4] = Interest payable

So let's try...

P = 0.028 x 1/4 x $14.39 NOPE!
P = 0.028 x 1/4 / $14.39 NOPE!
$14.39 / 1/4 ....

I dunno. Hint?

 

[Dr.Peterson]

What I can do in this area so far:
*figure out interest due when I know principle, interest rate (percentage), time

I'm facing two questions that ask me to figure out the principal, which seems to indicate all I have to do is the reverse, or divide something. But I can't think of what!

"Melissa's account earned $14.39 in 3 months. Simple interest rate is 2.8%. What was the principal?"

When I know principal, I do:

Principal x [rate as percentage, converted to demical] x time [if 3 months, 1/4] = Interest payable

So let's try...

P = 0.028 x 1/4 x $14.39 NOPE!
P = 0.028 x 1/4 / $14.39 NOPE!
$14.39 / 1/4 ....

I dunno. Hint?

Fill in what you know, as if you were finding the interest:

Principal * rate * time = Interest
______ * 0.028 * 1/4 = 14.39

You need to undo both multiplications. Or, if you prefer, do the one multiplication and undo the resulting multiplication.

(If you are comfortable with a little algebra, the blank would be a variable, and you would solve for it.)

 

[steveopolis]

Oh!

I didn't know how to undo both, so I went with Option B and did this:

P x 0.028 x 1/4 = $14.39

so...

0.028/1 x 1/4 = 0.028/4
0.028/4 = 0.007

14.39 / 0.007 = 2055.71

Which checks out!

$2055.71 x 0.028 x 1/4 = $14.39

Cool. Will now apply this to the similar questions. Thanks again, Dr.Peterson!

 

[Steven G]

What I can do in this area so far:
*figure out interest due when I know principle, interest rate (percentage), time

I'm facing two questions that ask me to figure out the principal, which seems to indicate all I have to do is the reverse, or divide something. But I can't think of what!

"Melissa's account earned $14.39 in 3 months. Simple interest rate is 2.8%. What was the principal?"

When I know principal, I do:

Principal x [rate as percentage, converted to demical] x time [if 3 months, 1/4] = Interest payable

So let's try...

P = 0.028 x 1/4 x $14.39 NOPE!
P = 0.028 x 1/4 / $14.39 NOPE!
$14.39 / 1/4 ....

I dunno. Hint?

I am glad that you got it in the end. I do want to point out that Simple interest rate is 2.8% makes no sense at all. One can reasonably ask if the 2.8% is early, every 3 month, every 4 month, etc. You need to ask precise questions which I am glad to say that you are trying to do.

 

[steveopolis]

I am glad that you got it in the end. I do want to point out that Simple interest rate is 2.8% makes no sense at all. One can reasonably ask if the 2.8% is early, every 3 month, every 4 month, etc. You need to ask precise questions which I am glad to say that you are trying to do.

Yeah, the language and content in the course make me cuckoo. For this series of questions, they did ask us to assume rate is per year, and to use fractions of 12 of months are expressed, and fractions of 365 if days.

I posted another question that I think doesn't make sense as is: https://www.freemathhelp.com/forum/threads/111400-proportion

Another asked me to state 20% depreciation after two years, but I don't know if that's asking me to figure out, for Year 2, 20% off the value found at the end of Year 1, or 20% per year off the original price. So I did both, and hope they accept the answer that matches the secret information found only in their head.