Only for solutions

Let $w$ be the number of hours he walks a certain distance and $r$ be the number of hours he rides the same distance. Then

$w+r=\dfrac{15}{4}$ $\color{#D61F06}(1)$

$r+r=\dfrac{5}{2}$ $\implies$ $2r=\dfrac{5}{2}$ $\implies$ $r=\dfrac{5}{4}$ $\color{#D61F06}(2)$

subsitute $\color{#D61F06}(2)$ in $\color{#D61F06}(1)$ , we have

$w+\dfrac{5}{4}=\dfrac{15}{4}$

$w=\dfrac{15}{4}-\dfrac{5}{4}=\dfrac{10}{4}=\dfrac{5}{2}$

Finally,

$2w=2\left(\dfrac{5}{2}\right)=$ $\boxed{5}$