1. Start both hourglass until 4-min glass hourglass gives the full reading. Now, time = 4 min. reading remained in 7-hg is 3 min. 2. Again start 4 hg from begining with 7-hg which has 3 min left. 3. After three min 7-hg completes its cycle. In 4 hg there is still 1 min reading is remained. Up to this time total time recorded is 7 min. 4. Start 7 hg glass again along with 4 hg having 1 min reading left. 5. After one min, cycle is completed for 4 hg. Now on the bottom part of 7 hg 1 min reading is remained. Upto here total time is 8 min. 1 more to go. 6. Reverse 7 hg to make 9 min.