A 10 cm long rectangular bar (when subjected to a tensile load) deforms by 0.1 mm. calculate the normal strain.

The strain is the ratio of the deflection (change in dimensions) to the original dimensions.

SO:

strain = 0.1 mm / (10*10 mm) = 0.1 %

Take care of the unit ... you must use the same units.

and as it a tensile load, so the strain would be positive ( + 0.1 %)