Failure rate of a system

问题:

In a Microsoft interview I was asked the following question:

A system is guaranteed to fail 10% of a time within any given hour, what's the failure rate after two hours ? after a million hour ?

I'm not very experienced in Reliability theory and Failure rates, but any input on this question will be very much appreciated.

回答1:

1-(.9^h), where h is the number of hours