Answer:
The answer is: 30 to 40%, according to satellite imagery
Explanation:
Satellite imagery is the most reliable way to try to determine what percentage of the Earth's land is used for agriculture. First and most important, the world is huge and only satellites have the technology to take images of continents and differentiate what possible use is given to them.
Government's surveys and records are no very reliable since you would need to use information from all the governments in the world including democratic countries and nations with dictatorships.