Treasury Bills, or T-Bills, are short-term investments backed by the United States government. These bills are sold at auction for a discount from their face value. When the T-Bills matures, the investor receives the face value. Any difference between the face value and his purchase price constitute the profit. When an investor submits an ask price, he's effectively making an offer to purchase the T-Bill at this price. The potential profit, when expressed as a percentage of the asking price, is called the ask yield.

Subtract the ask price from the face value of the T-Bill. The face value is typically $100. As an example, if you submitted an ask price of $97.50, then the difference would be $2.50.

Divide this difference by the asking price. In the example, $2.50 divided by $97.50 gives you 0.0256. This is the yield for the T-Bill maturity term, but the actual ask yield should be converted to an annual percentage, so you need to adjust for the T-Bill term.

Divide the T-Bill term by 365. In the example, if you were purchasing a 13-Week T-Bill, you would divide its 91 days by 365 to derive 0.2493.

Divide the term yield by this time fraction to calculate the ask yield. In the example, 0.0256 divided by 0.2493 gives you an ask yield of 0.1027, or 10.27 percent.

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