Explanation

Recognise the identity a³-b³ = (a-b)(a²+ab+b²) with a = 5.4 and b = 0.4, since 0.064 = 0.4³, 2.16 = 2x5.4x0.4 and 0.16 = 0.4². The fraction therefore equals a-b = 5.4-0.4 = 5.0. Hence the keyed option is C.