Percentage decrease is 33.5%.
$\text{% decrease} = \dfrac{ \color{blue}{ \text{initial value} } - \color{red}{ \text{final value} }}{ \color{blue}{ \text{initial value} } } \times 100$ | $$ \begin{aligned} \color{blue}{ \text{initial value} } &= 3.91 \\[1.5em] \color{red}{ \text{final value} } &= 2.6 \end{aligned} $$ |
$\text{% decrease} = \dfrac{ \color{blue}{ 3.91 } - \color{red}{ 2.6 } }{ \color{blue}{ 3.91 } } \times 100 $ | |
$\text{% decrease} = 0.335 \times 100 $ | |
$\text{% decrease} = 33.5 \% $ |