13. For the school library, 17 identical books were purchased. How much do they cost if for 9 such books they paid more than 11 p. 30 k., but less than 11 p. 40 k.?
13. It is known that 9 books were paid $\overline{113 x}$ k. Since this number must be divisible by 9, then $1+1+3+x=9$, from which $x=4$. Therefore, one book costs 126 k. $=1$ r. 26 k., and 17 books $-126 \cdot 17=2142$ k. $=21$ r. 42 k.