\(B=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}...\frac{19}{20}\)
\(B=\frac{1.2.3...19}{2.3.4...20}\)
\(B=\frac{1}{20}\)
\(B=\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}...\frac{19}{20}\)
\(B=\frac{1.2.3...19}{2.3.4...20}\)
\(B=\frac{1.\left(2.3.4...19\right)}{\left(2.3.4...19\right).20}\)
\(B=\frac{1}{20}.\)