You are given two hour glass which can measure 4 and 7 min respectively. now you have to calculate 9 min using these two hour glass (Note: you have to calculate it at a stretch)

lets say we have two hour glasses A and B and let’s keep there top and bottom time as (t,b) for our understanding.

so initially

A: (4,0) B: (7,0)

reverse A after it is over (i.e after 4 minutes)

A: (4,0) B: (3,4)

reverse B after it is over (i.e after 3 minutes)

A: (1,3) B: (7,0)

reverse A after it is over (i.e. after 1 minute)

A: (4,0) B: (6,1)

After A is over (i.e after 4 minutes)

A: (0,4) B:(2,5) —

Start your timer now and count 2 minutes from B hour glass and than reverse it after it is over and count 7 more minutes from it. thus you can count a total of 9 minutes at a stretch.