Radius of the air bubble,
r = 1 cm
Density of air, ρ = 0
Density of liquid (viscous medium)
σ = 0.8 cm-3
Terminal velocity of air bubble,
v = -0.5 cm s-1
(The velocity has taken negative value, for the bubbles is moving upwards).
Now,
v = {2(ρ - σ)r2g}/{9η}
η = {2(R - σ)r2g}/{9} = {2(0 - 0.08) x (1)2 x 890}/{9 x (-0.5)}
= {2 x 0.08 x (1)2 x 890}/{9 x 0.5}
= 348.44 Poise