Sand Glass II?

Question

A teacher of mathematics used an unconventional method to measure time for a test lasting 15 minutes. He used just a sand-glass, which spills in 7 minutes and a second sand-glass, which spills in 11 minutes. During the whole time he turned sand-glasses only 3 times. Explain how the teacher measured 15 minutes.

Answer 1

He started them both at the same time (2)

When the 7 minute glass finished, he still had 4 minutes remaining in the 11 minute glass.

Then when the 11 minute finished, he quickly turned it again (3) for a full term of 11 minutes.

He turned the two at the start and the one for a total of three.

There he had the remaining 4 minutes from the start plus the full 11 minutes.
.

Answer 2

1. Start both the Sand glasses.
2. Once the 7-minute glass spills over, start the test.
3. By the time 11-minutes glass spills over, we would have finished 4-minutes.
4. Once the 11-minutes spills over, turn it over again.

Robert has a flaw in his, if he flips both, waits for the 11 to finish, then flips it again for a full term, thats 22 minutes.

Answer 3

t= 0:
start both of them

t = 7:
turn over the 7

t = 11:
the 11 runs down, and the 7 has 3 mins to go, which means 4 mins have elapsed from the 7. So turn over the 7, and this will take you to 15 mins

Answer 4

he sets them both going.
When the 7minute hour is emptied , 7 minutes have elapsed and he now turns them both over. When the 11 minute glass is emptied 4 aditional mintes have elapsed for a total of 11 minutes now he turns the 7 minnute glass over and when it is empty another 4 minutes has elapsed for a total of 15 minutes.
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