Function calculate_simple_interest

calculate_simple_interest(
    principal: number,
    apr: number,
    months: number,
    digits?: number,
): number
Calculates the future value of the principal amount given the specified rate and the number of months in which the principal is accruing interest using the simple interest formula
Parameters
- principal: number
  the starting principal
- apr: number
  the annual percentage rate
- months: number
  the number of months in which the principal is held
- digits: number = 2
  the number of digits to round the result to
Returns number