Here, outer radius of each column
r0 = 60 cm = 60 x 10-2 m
Inner radius, ri = 30 cm = 30 x 10-2 m
Area of cross-section of each column,
A = πr02 -πri2 = π(r02 - ri2)
= 3.14 x [(60 x 10-2)2 - (30 x 10-2)2]
= 0.85 m2
Total mass supported by four columns,
= 50000 kg
Mass supported by each column,
= 50000/4 = 12500 kg
Stress on each column = mg/A = 12500 x 9.8/0.85
= 1.44 x 105 Nm-2
Now, Young's modulus, Y = 2.0 x 1011 Nm-2
Using the relation, Stress/Strain = Y
We have, Strain = Stress/Y = 1.44 x 105/2 x 1011 = 7.2 x 10-7