Fill the 11 gallon container from tank. Now tank will have 30 - 11 = 19 gallons of water. And from the that 11 gallon container fill the 7 gallon container. So, in 11 gallon container the water remained will be 11 - 7 = 4 gallons. Throw those 4 gallons empty the 11 gallon container. And the drop the water of 7 gallon container back in to the tank. So tank will have 19 + 7 = 26 gallons of water.

Now our both containers are empty and tank has 26 gallons. Fill the 11 gallon container from tank. Now tank will have 26 - 11 = 15 gallons. Tank is now half full.

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Bikhrunga Fizaun Mein Ek Noor Ki Manind . . .