The formula for volume is V = (L)(W)(H). Length, width, and height are multiplied together. In this case, height is thickness. Since we are given the total area, length, and width, we just need to solve for the height/thickness. Plug in the given values:
225 = (25)(18)(H) 225 = 450*H H = 225 ÷ 450 H = 0.5
