You have been given sand time clocks of 4 and 7 minutes. How to measure 9 minutes?

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7 min clock:|——-7——-|

4 min clock:|—–4—-|—-4—–|—–4—-|—–4—-|

you got 9 min: |—————9————–|

Start both clocks web 4 min is completed reverse it.

Now it’s 3 min and 4 min sand in both the clocks and 4 min is over till now

When 3 min more is over. 7 min clock is empty and 1 min sand is remaining in 4 min clock. Total time over till now=4+3=7

Reverse 7 min clock. After 1 min 4 min clock is empty and in 7 min clock 6 min sand is on upper side and 1 min sand on lower side. Total time over till now is 4+3+1=8.

Now reverse the 7 min clock. So 1 min is on upper side and 6 min on lower side. Calculate 1 min from here.

Total time=4+3+1+1=9