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?
As some of you have described already - you've got to start a load of timers at once. Have one at double speed - 30 sec, when this runs out there are 30 seconds left, hit the next timer at doule speed - it will run for 15 sec, when it runs out - hit doule on the next etc etc etc etc -
you need to do this for 49 sequential clocks. When you hit double speed on the 49th it will run for 2.13 x10(-13) seconds - that was the easy bit- now we need to get 1.17 x10(16) of them to run consecutively. Does this sound a bit complex? Is it more logical than this? Do you want me to go on or am I barking up the wrong tree??