Percentage decrease is 20.63%.
$\text{% decrease} = \dfrac{ \color{blue}{ \text{initial value} } - \color{red}{ \text{final value} }}{ \color{blue}{ \text{initial value} } } \times 100$ | $$ \begin{aligned} \color{blue}{ \text{initial value} } &= 0.63 \\[1.5em] \color{red}{ \text{final value} } &= 0.5 \end{aligned} $$ |
$\text{% decrease} = \dfrac{ \color{blue}{ 0.63 } - \color{red}{ 0.5 } }{ \color{blue}{ 0.63 } } \times 100 $ | |
$\text{% decrease} = 0.2063 \times 100 $ | |
$\text{% decrease} = 20.63 \% $ |