17) A leather ball is obtained by sewing 20 pieces of leather in the shape of hexagons and 12 pieces of leather in the shape of pentagons. A seam joins the sides of two adjacent pieces. Then the total number of seams is

$\begin{array}{llll}\text { (A) } 90 & \text { (B) } 172 & \text { (C) } 176 & \text { (D) } 180\end{array}$

(E) the data in the problem are insufficient.
17) The answer is $\mathbf{( A )}$.

Indeed, by counting the number of seams adjacent to the hexagons $(6 \cdot 20=120)$ plus the number of seams adjacent to the pentagons $(5 \cdot 12=60)$, each seam is counted twice, since it is adjacent to two polygons.