Treasury bills have a fixed face value (say $1,000) and pay interest by selling at a discount. For example, if a one-year bill with a $1,000 face value sells today for $950, it will pay $1,000-$950=$50 in interest over its life. The interest rate on the bill is therefore $50/$950=.0526 of 5.26 %.
Now generalize this example. Let P be the price of the bill and r be the interest rate. Develop an algebraic formula expressing r in terms of P.