Shikha's income is 60% more than that of Shalu.

Let Shalu's income be Rs $x$.

$\therefore$ Shikha's income $=\operatorname{Rs}\left(\mathrm{x}+\frac{60 x}{100}\right)=\mathrm{Rs} \frac{160 x}{100}=\mathrm{Rs} \frac{16 x}{10}$

Difference in the incomes of Shikha and Shalu $=\frac{16 x}{10}-x=\frac{16 x-10 x}{10}=$ Rs $\frac{6 x}{10}$

Percentage of the difference in the incomes of Shikha and Shalu to that of Shikha's income $=\frac{\frac{6 x}{10}}{\frac{16 x}{10}} \times 100=\frac{600}{16}$

= $37.5 \%$.

$\therefore$ The income of Shalu is less than that of Shikha by $37.5 \%$.