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.

Vishal Singh Chauhan says

1.Start both hour glass at the same time…

When the 4 min completes in 4min. Hour glass.. The 7min hour glass has still 3 minutes left..

2. Reverse the 4 min hour glass…and start comparing it with the 7 min hour glass… when the 3 mins completes which was left in 7min hour glass..the 4 min Hour glass has still 1 minutes left..

Start Counting the time from that moment…

(so we will get 1 minute in this way)

3. When the 1 min. Completes reverse the 4 min hour glass again… And when thus time the 4 min hour glass completes..

(U now have counted 5 mins in this way)

1min.+4min. =5min.

4. Now reverse the 4min hour glass again… And when it completes…

U now have counted 9 minutes.

1min. +4min.+4min. =9min.

SasiKanth Jam says

Start both 7 min & 4 min hour glass at the same time.

once 4 hour glass completes then reverse it, now 7 hour glass is at 4 min and 3 min remaining..

when 7 hour glass completes then reverse it. the reversed 4 hour glass still has 1 min to complete..

after you reverse 7 hour glass wait unitl the 4 hour glass to be completed thereby your total is 9 min.

irnathhingmire says

Start both glass at same time.

At 4 hour:

When 4 hour glass reaches its limit reverse it.

both glasees will be at 4 hour. The 7 hour glass remains 3 hour sand.

at 7 hour:

As soon as 7 hour glass emptied reverse it.

The sand remains in 4 hour glass is for 1 hour.

So both glass will run for another one hour.

At 8 hour:

now 4 hour glass will going to be emptied. As soon as it emptied it shows 8 hour. And the sand fall in 7 hour glass will also be of 1 hour.

Reverse the 7 hour glass immediately…it will run for another one hour. Thua we can count 9 th hour.