i have to create a program,but im not exactly sure what all im supposed to do, and what everything means.

The formula for computing the simple interest on a loan is calculated as

interest = principal * rate * days / 365

The ACME credit company needs a program that calculates the simple interest on customers’

purchases. Each customer may have one of the following types of credit cards: (P)latinum,

(G)old, (S)ilver, or regular. Each of the different types of credit cards corresponds to a different

interest rate.

• The regular rate is currently 18.75%.

• The rate for silver customers is 13.24%

• The rate for gold customers is 8.24%

• The rate for platinum customers is 3.99%

i need to compute simple interest. The programshould input the principal, rate, and days from the users. Your program should allow a user to enter the rate as a single letter corresponding to the type of the credit card.

thats it...my question is what is principle,and how would i even set this up?? how do i find it...or do i already have it? I have no clue where to begin on this project, and not knowing what principle is is'nt helping. could someone enlighten me?