Answer is: the thickness is 0.0256 millimeters.
m(Al) = 1275 g; mass of aluminium foil.
d(Al) = 2.7 g/cm³; density of aluminium foil.
V(Al) = m(Al) ÷ d(Al).
V(Al) = 1275 g ÷ 2.7 g/cm³.
V(Al) = 472.2 cm³ ÷ 10⁶ cm³/m³.
V(Al) = 0.000472 m³, volume of aluminum foil.
V(Al) = P · l (thickness).
l = 0.000472 m³ ÷ 18.5 m².
l = 0.0000256 m · 1000 mm/m.
l = 0.0256 mm.