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September 22, 2009

Hourglass puzzles

You have two sand hourglasses, one that measures exactly 4 minutes
and one that measures exactly 7 minutes. You need to measure out
exactly 2 minutes to boil an egg. Using only these two hourglasses,
how can you measure out exactly 2 minutes to boil your egg?


Varun said...

4 Min 7 Min
After 4 Min
Empty 3 Min remains
Reverse 1st glass
4 Min Remains 3 Min remains
After 3 Min
1 Min remains 0 Min remains
Reverse 2nd glass
1 Min remains 7 Min remains
After 1 Min
0 Min remains 6 Min remains
Reverse 1st glass
4 Min remains 6 Min remains
After 4 Mins
0 Min remains 2 Min remains

Now we can measure exactly 2 minutes when 2nd glass is empty.

Secret Squïrrel said...

A quicker solution:

4 : 7 Start both timers
0 : 3 After 4 min
4 : 3 Invert #1
1 : 0 After 3 min
1 : 7 Invert #2 and start cooking egg
0 : 6 After 1 min (egg half-cooked)
0 : 1 Invert #2
0 : 0 Stop cooking egg.

Total time from start: 7 min

Anonymous said...

wow... dats cool!!

Anonymous said...

how about you just wait for the 1st hourglass to be half way and that should be two minute.

Anonymous said...

You can't accurately tell when half of the sand has fallen thru as the pile on the bottom is a different shape from what's in the funnel.

Anonymous said...

No.1 = 4 mins
No.2 = 7 mins

3 X 4 = 12

2 X 7 = 14

Begin both hourglasses at the same time.

After 4 mins from start time (when no.1 empties), invert no.1

After 7 mins from start time (when no.2 empties), invert no.2

After 8 mins from start time (when no.1 empties again), invert no.1

Allow no.1 to fully empty once more, then immediately start boiling the egg. After exactly 2 mins, no.2 will empty, and the egg will be cooked.

Anonymous said...

Secret Squïrrel is correct.

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