Chapter 13 - A Preview of Calculus: The Limit, Derivative, and Integral of a Function - Section 13.4 The Tangent Problem; The Derivative - 13.4 Assess Your Understanding - Page 918: 53

$10$

We know that the combination formula is given by: $C(n,r)=\dfrac{n(n-1)(n-2)...(n-r+1)}{r!}$ This implies that $C(5,3)=\dfrac{5\cdot4...(5-3+1)}{3!}=\dfrac{5\cdot4\cdot 3}{3!}$ Hence, $C(5,3)=\dfrac{5\cdot4\cdot3}{3 \cdot 2 \cdot 1}=\dfrac{60}{6}=10$