Last year Martha could grow 20 bushels of basil or 25 bushels of cilantro on each acre of land that she farmed. This spring, she discovered a free organic fertilizer that allows her to double the amount of cilantro that she can grow on each acre, ceteris paribus. How has Martha's opportunity cost changed with this discovery

Martha's opportunity cost of growing cilantro over basil has decreased by 50%

Explanation:

Last year Martha's opportunity cost of growing cilantro over basil was 20/25 = 0.8 bushels of basil per bushel of cilantro. Since her productions possibilities frontier of cilantro has doubled this year, her opportunity cost of growing cilantro over basil is 20/50 = 0.4 bushels of basil per bushel of cilantro.