Partially fill the bottle and mark the water level. Invert the bottle and note the water level relative to the mark. Add/subtract water as necessary and repeat the steps until the water level is the same upright and inverted.
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its simple.first fill the bottle upto
some extent.and now invert the
 bottle mark the level.add the
water if the level is less when
inverted and subtract if the level
is more when inverted.