Then, subtract the resulting number from 100. That will give you the price of a Treasury bill with a face value of $100. If you want to invest more, then you can adjust the figure accordingly.
As a simple example, say you want to buy a $1,000 Treasury bill with 180 days to maturity, yielding 1.5%. To calculate the price, take 180 days and multiply by 1.5 to get 270. Then, divide by 360 to get 0.75, and subtract 100 minus 0.75. The answer is 99.25. Because you're buying a $1,000 Treasury bill instead of one for $100, multiply 99.25 by 10 to get the final price of $992.50.