That's not the right formula for the return you're after.

As I said, first find the price of the Bill at the two points in time. Divide the bigger by the smaller to get the nominal return - something like 1.03. Take that number and annualize it by raising it to the power of (365/n) where n is the number of days held. Then subtract 1.

It would look like this: (P2/P1)^(365/n)-1

Where P1 and P2 are price at time 1 and 2 respectively.

Actually I think that celebration may have been a little premature. Using the formula from above an driving the sale prices of the t-bills using 360 day basis and thne anualising it to a 365 basis I have come up have come up with the answer 7.636% which isn't a million miles away from 7.90% in real turn but mathamatically speaking its quite a way off so I am officially bamboozled because the examples of this sort of question in the study matieral are much more simple

For example one is as follows:

A UK tresury bill with remaining maturity of 70 days is quoted at a disxount rate of 7.1%. What is the equivalent yield. And the answer they have given is:

7.1%/(1 - (0.071 x 70/365)) = 7.198%.

So I am well an truely lost. Thank you for trying to help. As I'm sure by now your probably just as confused as I am, and appologise for being a pain in the preverbials!!