Methodology
Price = face value × [1 − (discount yield × days ÷ 360)].
Simple investment yield = (face value − price) ÷ price × year basis ÷ days.
Discount yield uses face value and 360 days. Simple investment yield uses the amount invested and a 365- or 366-day year.
Example: a 91-day bill with a 4.5% bank discount yield costs approximately $98,862.50 per $100,000 face and repays $100,000 at maturity.
This short-bill model is restricted to 182 days or less. Longer US Treasury bills use additional official investment-rate conventions that are not modeled here.
How to interpret the result
Why is investment yield higher than discount yield?
The bank discount yield uses face value as its denominator; investment yield uses the lower purchase price.
Reference
US Treasury – Understanding pricing and interest rates
Educational model. Inputs are illustrative; verify market conventions and contractual terms before use.