$$(1- \frac{ 1 }{ 2 } )(1- \frac{ 1 }{ 3 } )(1- \frac{ 1 }{ 4 } ).........(1- \frac{ 1 }{ 20 }$$
$\frac{1}{4}=0.25$
$$\left(\frac{2}{2}-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)$$
$$\frac{2-1}{2}\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)$$
$$\frac{1}{2}\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)$$
$$\frac{1}{2}\left(\frac{3}{3}-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)$$
$$\frac{1}{2}\times \frac{3-1}{3}\left(1-\frac{1}{4}\right)$$
$$\frac{1}{2}\times \frac{2}{3}\left(1-\frac{1}{4}\right)$$
$$\frac{1\times 2}{2\times 3}\left(1-\frac{1}{4}\right)$$
$$\frac{1}{3}\left(1-\frac{1}{4}\right)$$
$$\frac{1}{3}\left(\frac{4}{4}-\frac{1}{4}\right)$$
$$\frac{1}{3}\times \frac{4-1}{4}$$
$$\frac{1}{3}\times \frac{3}{4}$$
$$\frac{1\times 3}{3\times 4}$$
$$\frac{1}{4}$$
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$\frac{1}{2 ^ {2}} = 0.25$
