Measure 9 hours with hourglasses

puzzle : How will you measure 9 hours using only a 4-hour and 7-hour hourglass?

quite easy puzzle, can be done with hit and trial

Solution ==>

1) start with both the hour glasses fresh.

2) after 4 hours when A is empty , just turn A upside down

we have measured 4 hours by now

3) after 3 hours ,A will be empty and 1 hour sand will be remaining in A, just turn B upside down

we have measured 4+3 = 7 hours by now

4) after 1 hour when A is empty , B has 1 hour sand remaining
turn B upside down

we have measured 7+1 = 8 hours by now

5) after 1 hour B will be empty

we have measured 8+1 = 9 hours

6) done...