Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.
Q: Problem: rectangular floors x and y have equal area. if floor x is 10 feet by 18 feet and floor y is 9 feet wide, what is the length of floor y, in feet ?
Options: ['a. 13 1 / 2', 'b. 20', 'c. 18 3 / 4', 'd. 21', 'e. 24']
A:
b