you first have to find the area of the room, so the total square feet on the inside, then take that number and multiply by the amount per square foot m 13x15=195 (total sq ft of the room) 195 x 1.15 = 224.25
"20% off regular price" can be calculated by multiplying the regular price by (1.00-0.20), or 0.80. Thus, the discounted price will be 0.80($120), or $96.