42. Ten identical books cost no more than 11 p., while 11 such books cost more than $12 \mathrm{p}$. How much does one book cost?
42. Let one book cost $x$ k. Then we have $x \cdot 10 \leqslant 1100$ and $x \cdot 11 > 1200$ or $x \leqslant 110$ and $x > 109 \frac{1}{11}$. Therefore, $x=110$ k., i.e., the book costs 1 p. 10 k.