Poisson distribution - to find a time interval

Hi,

Wow I love preparing for my IB finals, because I'm on here constantly... trying to get some help... but to my question:

The following information is given: Casualties arrive at an accident unit with a mean rate of one every 10 minutes. Assume that the number of arrivals can be modelled by a Poisson distribution.

d. Calculate the time interval during which there is a 95% chance of there being at least two casualties.

At first the task seems pretty simple and I see that I have to formulate an equation to find the unknown, but the thing is I don't know exactly how do I do that. I mean p=0.95 so I guess that could be one side of the equation, but what to put on the other side? The poisson pdf function doesn't seem to provide any easy way out of it?

Thanks in advance.