Working with the Simple Interest Formula

The single relation $SI = \dfrac{PRT}{100}$ contains four quantities, so if any three of them are known the fourth can be found by rearranging the same formula. This is the most common type of examination problem in this part of the chapter, and there is nothing new to learn — only algebra applied to one formula.

Rearranged for each unknown:

When the amount is given instead of the interest, first recover the interest from $SI = A - P$, or work directly from $A = P\left(1 + \dfrac{RT}{100}\right)$. For instance, if a sum amounts to $A$ in $T$ years at rate $R$, the principal is

$P = \dfrac{A}{1 + \dfrac{RT}{100}}.$ …