Two sections in a bar of length 10cm and 20cm respectively are pulled axially. It causes an elongation of 0.2mm and 0.4mm respectively in each section. What will be the resultant strain on the bar?

The strain =$\frac{\mathrm{dL}}{\mathrm{L}}$

In a bar of different sections, the resultant strain is the algebraic sum of the individual stresses.
In column 1, strain = 0.2/100 = 0.002
In column 2, strain = 0.4/200 = 0.002
Resultant strain = 0.002 + 0.002 = 0.004