Percentage decrease is 95.83%.
$\text{% decrease} = \dfrac{ \color{blue}{ \text{initial value} } - \color{red}{ \text{final value} }}{ \color{blue}{ \text{initial value} } } \times 100$ | $$ \begin{aligned} \color{blue}{ \text{initial value} } &= 240 \\[1.5em] \color{red}{ \text{final value} } &= 10 \end{aligned} $$ |
$\text{% decrease} = \dfrac{ \color{blue}{ 240 } - \color{red}{ 10 } }{ \color{blue}{ 240 } } \times 100 $ | |
$\text{% decrease} = 0.9583 \times 100 $ | |
$\text{% decrease} = 95.83 \% $ |