Since we know that 1 foot is 12 inches, we can determine that a half of a foot is 6 inches. We have 7 feet, so we need to multiply 12 and 7 to find 7 feet in inches. 12*7 is 84, but we still have to add in our additional 6 inches. When we add 6 to 84, we get 90. Therefore, the classroom door is 90 inches tall.