\frac{1}{2} \left( \dbinom{6}{0} + \dbinom{6}{1} + \dbinom{6}{2} + \dbinom{6}{3} \right) = \frac{1}{2} (1 + 6 + 15 + 20) = \frac{42}{2} = \boxed{21}

Wed, 06 May 2026 18:02:29 +0000

Content under review