Rumus	\(N\, \sim Negative\,Binomial\,(r\,;p)\) \(E[N] = r\,\frac{q}{p}\, = 20\) \(Var[N] = r\,\frac{q}{{{p^2}}}\, = 24\)
Step 1	\(Var[N] = \left( {r\,\frac{q}{p}} \right)\,\frac{1}{p}\) \(24 = \left( {E[N]} \right)\frac{1}{p}\) \(24 = \left( {20} \right)\frac{1}{p}\) \(p = \frac{5}{6}\)
Step 1	\(q = 1 – p\) \(q = 1 – \frac{5}{6}\) \(q = \frac{1}{6}\)
Step 2	\(E[N] = r\,\frac{{\left( {\frac{1}{6}} \right)}}{{\left( {\frac{5}{6}} \right)}}\,\) \(20 = r\,\frac{1}{5}\,\) \(r = 100\)
Jawaban	e. 100