Radius of inner ring: 3cm/2 =1.5cm.

The number of layers of this film is: (2.5- 1.5)÷0.05=20 (layers).

The length of this membrane should be: (3 π+5 π)÷2×20=80 π (cm).

Namely: 80 π (cm) ≈25 1 cm.