Question: how many square feet is in a 20 by 20?
Answer: 20 inches is 20/12 = 1.666 feet so 20 inches by 20 inches is 1.666 × 1.666 = 2.778 square feet and 90/2.778 = 32.4 so it will take at least 33 tiles but I can't tell you the exact number.