The answer is the unknown material is the aluminium.
The density (D) is the quotient of mass (m) and volume (V):
D = m/V
The density of the unknown material:
m = 54 g
V = 20 cm³
D = 54 / 20 = 2.7
Since the density of the unknown material is 2.7 and the density of the aluminium is 2.7, the unknown material is the aluminium.