Puzzle Time

You have a couple of glass sand timers. One takes seven minutes for the sand to trickle through and the other takes eleven minutes for the sand to trickle through. Using these two, can you time a 15-minute passage of time, exactly?

(Taken from Johnny Ball's book - Ball of Confusion)

This reminds me of those quizzes on Professor Layton (not everyone may have heard of that) I don't think I could get it right without any help but will mull it over

I only can get 14.

My hands look like I'm doing a 'What's My Line' mime.

EDIT Sorted

7 + 8

Turn both over when seven has passed turn the 11 one back and when that has finished turn it back for the 11 mins ,

4 plus 11 is 15 mins

Imagine the glasses on a swivel, turning them horizontal stops them.

Use the 'seven' to time the 'eleven'

ie stop the 'eleven' leaving four mins sand left (7><4)

Then use that four mins to measure the 'seven'

ie after four minutes, the 'seven' is seperated into four and three minutes. (4><3)

Now you can do the 15 minutes

Use the four minutes, then the 'eleven'

Here's Johnny's answer:

"To time 15 minutes exactly, start both sand timers running.

When the 7-minute timer has emptied, turn it over.

When the 11-minute timer has emptied, turn the 7-minute timer over again.

The 7-minute glass will have just run 4 minutes since you last turned it, so it will take 4 minutes to run back again - finish on 15 minutes exactly."

Johnny's and Orrellite's way is quicker than mine.

You got there in the end though.

(My answer came from the end of the book.)

Veg Grower - my children have got the Prof Layton Nintendo DS games. They sometimes ask me for help, but not often.

easy peasy

Lemon squeezy?

My son's just solved it - Johnny's way - in a matter of minutes, but he is quite mathematically minded.

I completed the first Professor Layton, just not got round to doing the others yet. There is something slightly sinister about those - I love 'em.

I'm not a genius at Maths, but I was always well ahead in English. It would have taken me ages to work out that puzzle yesterday.

more puzzles needed

Taken, again, from Johnny Ball's book - Ball of Confusion:

7. I have three pairs of socks - a blue pair, a green pair and a red pair. I want to hang them on a washing line, but in a puzzling mathematical way.

I want to hang them so that:

There is 1 sock between the blues
There are 2 socks between the greens
There are 3 socks between the reds.

In what order on the washing line should I place them?