A steel scale measures the length of a copper wire as 80.0 cm, when...

Using the relation $l_t=l_0(1+\alpha t)$$\begin{aligned} & =1 \times\left[1+11 \times 10^{-6} \times\left(40^{\circ}-20^{\circ}\right)\right] \\ & =1.00022 \mathrm{~cm}\end{aligned}$Now length of copper rod at $40^{\circ} \mathrm{C}\begin{aligned} R_t{ }^{\prime} & =l_0{ }^{\prime}\left(1+\alpha^{\prime} t\right) \\ & =80\left[1+17 \times 10^{-6}\left(40^{\circ}-20^{\circ}\right)\right] \\ & =80.0272 \mathrm{~cm}\end{aligned}$$Now number of$cms observed on the scale =$\frac{80.0272}{1.00022}=$80.0096