You have an infinite amount of timers, each is an hour long (they do not have dials on telling you how long they've been going - they just beep when the time is up). You can set it to double speed at any time, but you cannot set it back to normal speed (eg if you set it to double speed at the start it will last 30 minutes.

Using each timer only once, is it possible to time exactly 25 minutes?

If it is, what is the smallest number of timers you need to do this, and the quickest time you can acheive it?

(In reply to

thoughts by Charlie)

Interesting. I don't have a lot of time to look at this now either, but I expect you are correct that no finite number of clocks will work. But he did say we have a infinite number to work with.

(I can see the whole "Marbles Bonanza" thread starting up again....)

So I expect the answer he has in mind is that you can do it if you use an infinite number of timers.

The interesting part is going to be "what is the quickest time" ie, what is the actual method used.