HELP!!! Hourglass Figures

7m - the seven minute clock
4m - the four minute clock
Turn both around. After the 4M runs out, turn both around again. After the 7m runs out, turn both around, and when the 4m runs out, turn around once more.
After the 7m runs out, 9 minutes have passed.

I'll post the complete explanation when you give up.

Relative

There's another way.

Start 4m and 7m.

When they run out turn them over.

When 4m runs out tun over 7m.

When that's done you'll have 9 minutes.

Craven, that doesn't work ..

Yes it does.

>>Start 4m and 7m.

>>When they run out turn them over.
4 minutes have passed. (0/4 , 3/7 is the remainder in them)
( The other option, where they both run out, does not work since you only measured 7 minutes, and are back at the start with 4/4 and 7/7).
When you turn them over, you have
4/4m, 4/7m

When 4m runs out tun over 7m.
8 minutes have passed. 7m is now empty on top, as is 4m.
When you turn over 7m, it has 7 on top.


When that's done you'll have 9 minutes.
No, you'll have 8+7=15 minutes

Neither of those answers will measure 9 minutes.

Relatives answer measures 16 minutes.

Cravens answer doesn't make sense.



They run out at different times.

First you turn both over.
When 4m is done turn it over. Thats 4 minutes.
When 7m is done turn BOTH over. That gives 7 minutes with 1 minute left in 4m.
When 4m is done turn them BOTH over. That gives 8 minutes with 1 minute left in 7m.
When 7m is done 9 minutes have passed.

Thus far, no problems.



This is where you are getting confused, because the wording is imprecise. It could mean waiting for both to finish before flipping but the other meaning is the intended one. Flip each as soon as they run out.



The 7m will have been flipped 1 minute before the 4m runs out (4+4=8-7=1).



8+1= 9

Yes, it does work.

You are confusing the inability to communicate this sense (or understand it) with the existence of said sense. I've explained my answer, it works perfectly and this is a very basic riddle that shouldn't be so confusing.

Adrian, your answer is correct, but not the shortest.

You're right, Craven
I didn't get your meaning.

my first answer is wrong

Yes, Cravens answer uses 2 less flips than mine. Damn.

How about;

First, flip both hourglasses over.
When the four-minute timer runs out, flip it again.
When the seven-minute timer runs out, flip it over
Now when the four-minute timer runs out again (after eight minutes) flip the seven-minute timer back over.
Since the seven-minute timer has been running only a minute between flips, there is a minute worth of sand left. Moreover, when that minute runs out it will be exactly nine minutes.

Yeah, that's a clearer way of saying what I'd said twice. Let's use that one!

just take yur watch and time
tryagain's ans. easiest