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?

why not just set one going on double speed and then when that reaches exactly five minutes, set another one at double speed for an hour which should stop after 30 minutes. The first timer should beep after its half an hour, if the other one that was put on is stopped as soon as the first one beeps, that will be on exactly 25 minutes, will it not????? So i think the answer is 2 timers