15 mins

Single Cell

If you have two sand filled timers,one for 11 minutes, and
one for 7 minutes, how could you use them to time 15 minutes without
wasting any time at the start?

D-Generate

Turn them both over so they start pouring out. When the 7 minute timer runs out there will still be 4 minutes left in the 11 minute timer's top half. Simply let this 4 minutes run out and then turn it back over again to get the full 15 minutes.
11-7 = 4
4 + 11 = 15

Single Cell

D-Gen..... Thanks 4that!!

Dman_15

D-Generate wrote:

Turn them both over so they start pouring out. When the 7 minute timer runs out there will still be 4 minutes left in the 11 minute timer's top half. Simply let this 4 minutes run out and then turn it back over again to get the full 15 minutes.
11-7 = 4
4 + 11 = 15

Surely this method has 7 minutes wasted at the start which aren't part of the 15.......

Single Cell

Dman 15..... you correct!! 7min are wasted!... it's a tricky one.

w66w66

Turn them both around to start timing.
1: After the 7 minute timer runs out immediately turn it back around again.
2: Then when the 11 minute timer runs out four minutes will have gone on the 7 minute timer, so immediately after the 11 minute timer has run out flip around the 7 minute timer.
3: Then let the seven minute timer run out.

Bill McH

Turn them both over so that the sand starts pouring. After 7 minutes, when the sand has run out in the 7-minute timer, immediately turn this over. After a further 4 minutes, the sand in the 11-minute timer will run out and there will be 4-minutes worth of sand in the bottom of the 7-minute timer. So if you turn this one over as soon as the 11-minute timer runs out, the time taken for that sand to drop will bring you up to 15 minutes - with no time wasted at the start. (sorry I can't make it more succinct).

Bill McH

Beaten to it!