SOLUTION: I need this answer quickly please: You have 2 hourglasses-a 4 min. hourglass and a 7 min. hourglass. You want to measure 9 min. How do you do it? Thanks

Question 20229: I need this answer quickly please:
You have 2 hourglasses-a 4 min. hourglass and a 7 min. hourglass. You want to measure 9 min. How do you do it?
Thanks
Answer by wuwei96815(245) (Show Source): You can put this solution on YOUR website!
Do you mean to measure 9 minutes as a continuous measurement? For instance, from 12:00 to 12:09. Or do you mean to measure 9 minutes in total? For instance, 3 + 3 + 3 without regard to continuity.

I think that I have an answer for you.

First, get both timers flowing at the same time. When the 4 minute timer runs out of sand, turn over both timers. When the 7 minute timer runs out of sand, turn over both timers again. At this point there should be 1 minute of sand left in the 4 minute timer. When that runs out, turn it over immediately (at this point you can disregard the 7 minute timer). When it runs out of sand again, turn it over for the last time. The last three runs of the 4 minute timer will add up to 9 minutes (1+4+4=9).

I hope that is the right answer.
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