Dr. Johnson has 2 sand-watches : one 7-minute and the second one 4-minute.
How can he correctly time 9 minutes?
See the answer
Dr. Johnson has to turn both sand-watches. After 4 minutes he has to turn upside down the 4-min sand-watch. When the 7-min sand-watch spills the last grain, he has to turn the 7-min upside down. Then he has 1 minute in the 4-min sand-watch 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-watch upside down and let the 1 minute go back. And that’s it. 4 + 3 + 1 + 1 = 9