The dissociation constant of a weak acid is  $1\times {10}^{-4}$ In order  to prepare a buffer solution with  a pH = 5 [Salt]/[Acid] ratio should be 

$$\begin{gathered} \mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \\ 5=4+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]}\left[\because {\mathrm{pK}}_{\mathrm{a}}=-\log {\mathrm{K}}_{\mathrm{a}}\right] \\ \text{ Given, }{\mathrm{K}}_{\mathrm{a}}=1\times {10}^{-4} \\ \therefore {\mathrm{pK}}_{\mathrm{a}}=-\log \left(1\times {10}^{-4}\right)=4 \end{gathered}$$
Now from Handerson equation 

$5=4+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]}$

$log\frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]}=5-4=1$
Taking antilog 
[Salt]/[Acid] = 10= 10: 1