The Americans with Disabilities Act (ADA) gives businesses and cities requirements, rules, for making sure that people in wheelchairs and scooters can use slopes. The ADA requires a 1:12 slope for wheelchairs and scooters. (This means that for every one unit of vertical distance, the ramp must have 12 units of horizontal distance.) Mr. Lee has a small business. He wants to make sure he builds the entrance to his building correctly. The entrance to Mr. Lee's business is 18 inches higher than the parking lot in front of his store. According to the ADA rules, how long does the ramp need to be?