Step 1: Understanding bulk modulus definition.
Bulk modulus is defined as:
\[
B = \frac{\text{stress}}{\text{volumetric strain}}
\]
For a cube, volumetric strain is related to linear strain by:
\[
\frac{\Delta V}{V} = 3 \frac{\Delta L}{L}
\]
Step 2: Calculating linear strain.
Initial length = 90 cm, final length = 89.5 cm:
\[
\Delta L = -0.5 \, \text{cm}
\]
\[
\frac{\Delta L}{L} = \frac{-0.5}{90} = -\frac{1}{180}
\]
Step 3: Calculating volumetric strain.
\[
\frac{\Delta V}{V} = 3 \times \left(-\frac{1}{180}\right) = -\frac{1}{60}
\]
Step 4: Applying stress value.
Given stress:
\[
P = 2 \times 10^9 \, \text{N m}^{-2}
\]
Step 5: Calculating bulk modulus.
\[
B = \frac{2 \times 10^9}{1/60} = 2 \times 10^9 \times 60 = 1.2 \times 10^{11}
\]
Step 6: Final conclusion.
\[
\boxed{1.2 \times 10^{11} \, \text{N m}^{-2}}
\]