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
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
7 min clock:|——-7——-|
4 min clock:|—–4—-|—-4—–|—–4—-|—–4—-|
you got 9 min: |—————9————–|