Notice that the initial expression can be rewritten as shown below
[tex]\frac{\frac{1}{2}\text{gallon}}{\frac{1}{8}\text{ minute}}=\frac{\frac{1}{2}}{\frac{1}{8}}\frac{\text{ gallon}}{\text{ minute}}=\frac{8}{2}\frac{\text{ gallon}}{\text{ minute}}=4\frac{\text{ gallon}}{\text{ minute}}[/tex]
The answer is 4 gallons per minute.
The mistake is that Jessie wrongly calculated (1/2)/(1/8) which should be solved using the next rule
[tex]\frac{\frac{a}{b}}{\frac{c}{d}}=\frac{ad}{bc}[/tex]
She did
[tex]\frac{\frac{a}{b}}{\frac{c}{d}}=\frac{ac}{bd}[/tex]
which is incorrect.