Feed the world is easy if we reduce meat consumption. Brazil, in this situation, has more then 70% of its agricultural lands producing meat (not considering lands unused), and almost 20% used to produce animal feed. The other 8-10% is used to produce vegetable, fruits, grains, whatever brazilians eat thats not meat.
10k m^2 of land used to produce meat is producing 54 kg/year in Brazil. The same to any fruit or grain goes from 2000 kg/year to 50000 kg. Some regions that produce meat using soybeans or corn produce about 200 - 300 kg/year (considering the land to keep the animal and the land to produce the grains).
(i could send links in portuguese if you want)
In the 90s meat that the US was eating was consuming enough to feed 800 million people.
(
http://www.news.cornell.edu/stories/1997/08/us-could-feed-800-million-people-grain-livestock-eat)
and we could feed 10 billion (Council for Science and Technology, How Much Land Can Ten Billion People Spare for Nature?, Feb. 1994, pg. 13.)
And these numbers are getting higher . Amazon forest is already becoming a barbecue.