This is a question of unit conversions. We are given the mass of a piece of aluminum foil, its area and the density of aluminum. The thickness is not given and has to be determined. We can solve this by converting all the quantities to a uniform set of units.
An ounce has 28.349 gm in it. Thus the mass of the aluminum foil is 8 x 28.349 gm = 226.792 gm.
A feet has 30.48 centimeters in it. Thus, the area of aluminum foil is 50 ft^2 or
50 x 30.48 x 30.48 cm^2 = 46451.52 cm^2.
Density is the ratio of mass and volume (which is the product of area and thickness). In other words,
Density = mass / volume = mass/(area x thickness)
Or, thickness = mass /(area x density)
Substituting all the values in the equation, we get
Thickness = 0.00181 cm = 0.0181 mm.
Hope this helps.