Answer:
The ratio of land to water in the northern hemisphere is about 1:1.5 or 2:31.
Thus the percentage of water cover out of total land area in the northern hemisphere can be found the following way 3/(2+3) = 0.6 or 60%.
The total surface area of the northern hemisphere is (255,036,000 km2)2. 60% of this area is 0.6 X 255,036,000 = 153,021,600 km2 = the amount of area covered by water in the Northern Hemisphere.
The percentage of land cover out of total land area in the northern hemisphere can be found the same way 2/(2+3) = 0.4 or 40%. 40% of this area is 4.6 X 255,036,000 = 102,014,400 km2 = the amount of area covered by land in the Northern Hemisphere.
Notice the ratio of land to water is 102 million km2 to 153 million km2, very close to the 1:1.5 ratio we began with.
The ratio of land to water in the south is about 1:41.
sources
(1) Pidwirny, Michael. "Ocean." eoearth.org. 05 June 2008. The Encyclopedia of Earth. 27 May 2009 <http://www.eoearth.org/article/Ocean>.
(2) ArthurWeasly, "Earth." Wikipedia.org. 27 May 2009. Wikipedia: The Free Encyclopedia. 27 May 2009 http://en.wikipedia.org/wiki/Earth.
About 61% of the northern hemisphere is covered by oceans.