48. Book Series. A certain series of books was published at seven-year intervals. When the seventh book was released, the sum of all the years in which the books of this series were published was 13524. When was the first book of the series published?
48. The arithmetic mean of the years of publication is $\frac{13524}{7}=$ 1932, or the middle term of our arithmetic progression. The first term of this progression differs from the middle term by three differences. Therefore, the first book was published in 1932 $-3 \cdot 7=1911$.

$[M$. M., 34, 372 (September 1961).]