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If you can wait 16 minutes then start 7 & 4 , reversing 4 when it is over.When 7 gets over, 4 will have one minute sand left.Use that plus twice 4 to get 9 minutes.Will be interesting to see it done in less time:-)
first start both 7 and 4 hour glasses(hg) after 4 mins 4hg will be emptyand time is 4magain reverse 4 hg after 3m 7hg is empty time is 4+3=7min reversed 4hg 1m sand is left also reverse 7 hg after 1m 4hg will get empty time is 7+1=8m and rev 7 hg as it has 1m sand revd for 9thmin 7hence calculated 9 mins
take 7 & 4 do both now when we hav claculated 4 mins we hav 3 mins remaning in 7 one now reverse both after 3 mins we hav 1 min remaining in 4th one now reverse 1 mins so we hav 5mins now add 4mins to it !!!!!!!!!!!!!!!!!!!!!!!
reverse both the sand glasses at a time, so that we get 3 mins.if we do this for 3 times we get 3+3+3=9