The amount of interest earned on A for one year is $336, while the equivalent amount of discount is $300. Find A.
You could create 2 simultaneous equations with 2 unknowns: $\displaystyle A$ and the interest/discount rate, call it $\displaystyle i$. The trick is putting $\displaystyle i$ in the right terms so it cancels when combining the 2 equations.
For example, one equation...
$\displaystyle A(1 + i) = 336$
After reading TKHunny's response, I see I left a piece of my equation out!! Previous should be...
$\displaystyle A(1 + i) = A + 336$
Sorry.
Hopefully this heads you in the right direction...?
-Scott
The important idea here is the difference between interest and discount processes.
Let's talk about short-term loans.
For Interest, you get A and pay back (A+Interest) later.
For Discount, you get (A-Discount) and pay back A later.
This leads to two structures. If 'i' is the rate if interest or discount...
Interest: A(1+i) = A + Interest
Discount: (A - Discount)(1+i) = A
In this case, we know two of the values, the Interest and the Discount:
Interest: A(1+i) = A + 336
Discount: (A - 300)(1+i) = A
Solve for A and i.