The real wordsTheory
The yield formula, both ways it is written
The Drive deck writes it generally, for a bill bought and sold at any two prices:
YT = ((SP - PP) / PP) x (365 / n)
The professor's own deck writes the Indian special case, where the selling price is par at 100:
Y = ((100 - P) / P) x (365 / D) x 100
They are the same formula. SP is the selling or maturity price, PP the purchase price, and n or D the number of days the bill is held.