In the 1800s and early 1900s, farmers and scientists were just learning that cross-breeding corn caused heterosis, allowing farmers to grow more corn per acre of land farmed. Two different varieties of corn were crossed reciprocally: Variety A yielded 20 bushels per acre and Variety B yielded 25 bushels per acre, and the resulting hybrid progeny corn produced 27 bushels per acre. What is the percent heterosis?