A sandclock question

If you have 2 sandclock . One of the clock is 11 minute and one is 7 minute . How can you set time of 15 minute ?

you turn both of them over, when the seven minute one runs out then u turn that one over again, then when the eleven minute one is finished turn over the seven minute one again. That currently has 4 minutes worth of sand in it, once that runs down its been fifteen minutes. 11+4=15

♥♦♣♠
.........................A..........B

start at.............11..........7
....zero............===.......===
minute..............0...........0

after..................4...........0
...7..................===.......===
minutes.............7...........7

..........invert.......4...........7
..........clock......===......===
.............B...........7...........0

after...................0...........3
..11..................===.......===
minutes............11...........4

..........invert.......0...........4
..........clock......===......===
.............B..........11.........3

after...................0...........0
..15..................===.......===
minutes............11...........7

start both going.
when the 7minute runs out, turn it over ===7 minutes
when the 11 rounds out (11 minutes) turn the 7 over (even though not finished)
when the 7 runs out, you've measured 15 minutes