This picture is taken from Problem Pictures Themes , a CD-ROM of photographs with activities for mathematics teaching.

Egg timer

When you turn over this egg timer, three sand timers measure 3 minutes, 4 minutes and 5 minutes. They cannot be turned over individually.

How could you use the timer to measure 6 minutes?

You could measure 7 minutes by adding together 3 and 4 minutes - but does it matter which you do first?

Is it possible to measure 2 minutes? Is it possible to measure half a minute?

What other times could you measure? What if you had three separate sand timers?