13.063. In the club's auditorium, there were 320 seats arranged in identical rows. After the number of seats in each row was increased by 4 and one more row was added, the auditorium had 420 seats. How many rows are there in the auditorium now?
Solution:

Let there be $x$ rows in the auditorium; $\frac{320}{x}$ - the number of seats in one row. After the number of seats in a row was increased by 4 and one row was added, the total number of seats in the hall became $\left(\frac{320}{x}+4\right)(x+1)=420$, from which $x=20$. Therefore, there are now 21 rows in the auditorium.

Answer: 21.