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?

12 Timers.

Set 2 timers T1 at normal speed and T2 at double. When T2 beeps 30 mins has elapsed. Set T1 to double at the beep and start T3, T1 will run for 15 mins and beep(a total of 45 mins ).When T1 beeps set T3 to double and start T4. T3 will then run another 22.5 mins(a total of 37.5 mins). When T3 beeps set T4 to double and start T5 at normal....etc.. ect.. The time elapsed from the beep of T11 till the beep of T12 is 25 mins exactly with no seconds left over or under.

Posted by kravix on 2003-09-24 18:58:20