Having 2 sand glasses: one 7-minute and the second one 4-minute, how can you correctly time 10 minutes?

• let glass 1 take 7 min
  glass 2 take 4 min
  set the two glasses
  after 4 min:glass 2 will b empty
  glass1:leave it undistubed
  glass 2:turn upside down
  it takes 3 more min for glass 1 to become empty
  after 7 min:glass 1 will be empty
  glass1:leave it undistubed
  glass2:turn upside down(it has completed 3 min now)
  so after 3 min glass 2 will be emptied
  4+3+3=10
• 12 years agoHelpfull: Yes(1) No(0)
• Turn both sand-glasses. After 4 minutes turn upside down the 4-min sand-glass. When the 7-min sand-glass spills the last grain, turn the 7-min upside down. Then you have 1 minute in the 4-min sand-glass left and after spilling everything, in the 7-min sand-glass there will be 1 minute of sand down (already spilt). Turn the 7-min sand-glass upside down and let the 1 minute go back. And that's it. 4 + 3 + 1 + 1 = 9
  
  
• 12 years agoHelpfull: Yes(0) No(0)