Construct AB of length 7.8 cm. From this, cut off AC of length 4.7 cm. Measure BC.

Given that $\overline{AB}$ = 7.8 cm and $\overline{AC}$ = 4.7 cm.
Step I: Place the zero mark of the ruler at A.
Step II: Mark a point B which is 7.8 cm away from A.
Step III: Mark another point C which is 4.7 cm away from A such that AC = 4.7 cm.
Step IV: On measuring the length of BC, we find that $\overline{BC}$ = 3.1 cm.