Lots of good answers sent in for this one! Well done everyone! There were three types of solutions.
Method A. Turning the 7-minute timer three times
Grace (Tao Nan School, Singapore) said this ...
11-7=4. This makes the leftover sand in the 11-minute timer 4 minutes.
In that case, when the 11 minutes are over, the 7-minute timer will have
3 minutes worth of sand left. 7-3=4, so 11+4=15 minutes.
Will and Scott (Antingham Southrepps Primary, Norfolk) said:We think that you should set the timers off at the same time ,
And here's one from Anna (Braga, Portugal)
George (Rosebank Primary School, Leeds) sent in a good explanation too.
Method B: Delay the start of cooking
Alistair (Tao Nan School, Singapore) sent in this solution:Firstly start both the 7-minute and the 11-minute timer.
Since 4 + 11 = 15, turning off the cooker when the 11-minute timer runs out of sand will cook the pasta for exactly 15 minutes!
Tom, Robert and Carly (Histon Junior School, Cambridge) say:
Daniel (Anglo-Chinese School - Primary, Singapore) thought about it the same way and sent in another good table. Thomas, Kimberley, Catherine, Matthew and Justin, all of Tattingstone School, each explained the same idea in their own way. Well done - they were all very clearly written.
Method C: Stop the timer
Celia (Roundwood Primary, Harpenden, Hertfordshire) found another way:
Start by turning over the 7 timer and the 11 timer together.
When the 7 timer stops, turn the 11 timer on it's side, because it has 4 mins. on it.
Immediately turn up the 11 timer and use the remaining 4 mins.
Then use it again (11 mins.)
11 + 4 =15
You have done 15 mins.
FOOD'S ON THE TABLE.