Ordinary simple interest is based on how many days in one banker’s year?

The correct answer is based on the convention used in banking and finance where ordinary simple interest calculations are typically done using a 360-day year. This practice simplifies interest calculations, especially when determining monthly interest, as it allows for a consistent approach to calculating the daily interest rate, where each month is assumed to have 30 days (12 months x 30 days = 360 days).

In contrast, the other options reflect different standard calendar days—365 days acknowledges a full calendar year, 366 days accommodates a leap year, and 365 ¼ days provides an average over several years considering leap years. However, for ordinary simple interest specifically, the 360-day year is the traditional standard that is most frequently applied in financial calculations. This convention enables more straightforward computation and consistency across various financial products.