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.

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.

anonymous · Answer

dont understand 
thts 29

anonymous · Answer

wow, one i know how to figure out from the get-go, thats rare lol, im writing a solution out right now.

anonymous · Answer

gotta keep busy whilst invigilating i guess

anonymous · Answer

its because 11 and 7 are coprime that you can right them in the form:
11a + 7b = 1 for some integers a and b.  Then you multiply that by 15.

anonymous · Answer

it will tell you how many times to use each glass.

anonymous · Answer

7 and 11 turn only 7, when 7 min left ( in 11 left 4 min) = 7 min
when 4 min left (11 empty ) in 7 left 3 min - turn 7 = 4 min 
4 min in 7 = 4
7+4+4=15

anonymous · Answer

thanks tatiana

anonymous · Answer

i mean to say is that first u make 11 min fall and 7 min when 7 min is done then u know tht time for the test strtd ... when 11 min is done then make it fall once again

anonymous · Answer

OK