You have two hourglasses. One empties in 7 minutes. The other empties in 4 minutes. How can you use these two hourglasses to measure exactly 9 minutes?
Flip both timers over. When the 4-min timer is empty, flip it over (the 7-min timer has 3 minutes left in the top). When the 7-min timer is empty, flip it over (there is now 1 minute left in the top of the 4-min timer). When the 4-min timer is empty for the second time, the 7-min timer has 1 minute worth of sand in the bottom half. Flip it over so there is now 1 minute in the top. When the 7-min timer empties, 9 minutes have elapsed.
|Step||Elapsed Time||4 Min Timer||7 Min Timer|
|2||0:04||FLIP||3 min left|
|3||0:07||1 min left||FLIP|
|4||0:08||FLIP (1 min now in top)|