Question 78918

OK i will try to answer this one since I like answering and since my dad once tried to teach me about compound interest.  Compound interest is not involved in this problem and it is hard to get a credit card company to give you their formula I think.

The problem says, as I see it, 
  X is invested at 10%
 2X is invested at 8%
The total interest is $91
What is invested at 8% ?

where X is "a certain sum that is invested".

What you will do is solve for X using the given information, and then as the final anwer give 2x since that is the quantity requested in the problem statement.

Generally investment rates are computed per year, so I am assuming the investment period they are talking about is 1 year, and I don't have to worry about compounding it where you'd compute (1.10)^N (N is the number of years) or (1.08)^N either.  They are just talking about some situation where you put your money in and later you get back 10% or 8% more and then the deal is done.

So you have X * (1.10) + 2X * (1.08) = 91 as your equation to solve

It helps to know when they say "percent" they are talking about hundredths.  There are ten fingers on a person's hands, ten pennies in a dime, and a hundred pennies in a dollar, and a hundred centimetres in a metre, so being human and living on planet Earth means you know "percent" means a hundredth.  Ten percent is written as 0.10 or 10/100 and when you earn ten percent that is added to 1 which is 1.00 so you can see 1.00 + 0.10 = 1.10 which is the original amount plus ten per cent interest.

Now you have 3.26 X = 91 

I am using my calculator to divide 91/3.26 but you could also use long division

X = 91/ 3.26 = 27.92  and I have rounded up from 27.913 because I want to have at least $91 when I am done, not less.  Your teacher may want you to round to X=27.91 instead.

The answer to the problem is closer to 55.83

Check this: 
55.83 plus 8 percent = 60.30  (55.83 * 1.08)
27.91 plus 10 percent = 30.70  (27.91 * 1.10)

Total is 91.00