Dinitrogen pentoxide decomposes into nitrogen dioxide and oxygen gas. the reaction is first order in dinitrogen pentoxide and has a half-life of 2.81 h at 25 celsius. if a 1.5l reaction vessel initially contains 745 torr of dinitrogen pentoxide at 25 celsius, what partial pressure of oxygen gas will be present in the vessel after 215 minutes?

Here is the explanation with the way of step-by-step that will definitely make it clear for you. 
At first, you have to make a balanced equation, as the one of the most essential steps.
The next step is to determine ICE chart with its relativeness to partial pressures.
And the last step that will sort things out is that  final partial pressure of the N2O5 that is done from the task will be related to how many h-lives it has benn through.
I am pretty sure it has become clear and you can solve it easily! Regards.