So, in order to solve this problem, you need to get the variable by itself:
1. Cross Multiply (if needed)
[tex] \frac{4}{x} = \frac{1}{5} [/tex]
[tex]x * 1 = 4* 5[/tex]
[tex]1x = 20[/tex]
2. Divide both sides by 1
[tex] \frac{1x}{1} = \frac{20}{1} [/tex]
[tex]x = 20[/tex]
